definition of hypothesis in genetics

Hypothesis n., plural: hypotheses [/haɪˈpɑːθəsɪs/] Definition: Testable scientific prediction

Table of Contents

What Is Hypothesis?

A scientific hypothesis is a foundational element of the scientific method . It’s a testable statement proposing a potential explanation for natural phenomena. The term hypothesis means “little theory” . A hypothesis is a short statement that can be tested and gives a possible reason for a phenomenon or a possible link between two variables . In the setting of scientific research, a hypothesis is a tentative explanation or statement that can be proven wrong and is used to guide experiments and empirical research.

It is an important part of the scientific method because it gives a basis for planning tests, gathering data, and judging evidence to see if it is true and could help us understand how natural things work. Several hypotheses can be tested in the real world, and the results of careful and systematic observation and analysis can be used to support, reject, or improve them.

Researchers and scientists often use the word hypothesis to refer to this educated guess . These hypotheses are firmly established based on scientific principles and the rigorous testing of new technology and experiments .

For example, in astrophysics, the Big Bang Theory is a working hypothesis that explains the origins of the universe and considers it as a natural phenomenon. It is among the most prominent scientific hypotheses in the field.

“The scientific method: steps, terms, and examples” by Scishow:

Biology definition: A hypothesis  is a supposition or tentative explanation for (a group of) phenomena, (a set of) facts, or a scientific inquiry that may be tested, verified or answered by further investigation or methodological experiment. It is like a scientific guess . It’s an idea or prediction that scientists make before they do experiments. They use it to guess what might happen and then test it to see if they were right. It’s like a smart guess that helps them learn new things. A scientific hypothesis that has been verified through scientific experiment and research may well be considered a scientific theory .

Etymology: The word “hypothesis” comes from the Greek word “hupothesis,” which means “a basis” or “a supposition.” It combines “hupo” (under) and “thesis” (placing). Synonym: proposition; assumption; conjecture; postulate Compare:   theory See also: null hypothesis

Characteristics Of Hypothesis

A useful hypothesis must have the following qualities:

It should never be written as a question.
You should be able to test it in the real world to see if it’s right or wrong.
It needs to be clear and exact.
It should list the factors that will be used to figure out the relationship.
It should only talk about one thing. You can make a theory in either a descriptive or form of relationship.
It shouldn’t go against any natural rule that everyone knows is true. Verification will be done well with the tools and methods that are available.
It should be written in as simple a way as possible so that everyone can understand it.
It must explain what happened to make an answer necessary.
It should be testable in a fair amount of time.
It shouldn’t say different things.

Sources Of Hypothesis

Sources of hypothesis are:

Patterns of similarity between the phenomenon under investigation and existing hypotheses.
Insights derived from prior research, concurrent observations, and insights from opposing perspectives.
The formulations are derived from accepted scientific theories and proposed by researchers.
In research, it’s essential to consider hypothesis as different subject areas may require various hypotheses (plural form of hypothesis). Researchers also establish a significance level to determine the strength of evidence supporting a hypothesis.
Individual cognitive processes also contribute to the formation of hypotheses.

One hypothesis is a tentative explanation for an observation or phenomenon. It is based on prior knowledge and understanding of the world, and it can be tested by gathering and analyzing data. Observed facts are the data that are collected to test a hypothesis. They can support or refute the hypothesis.

For example, the hypothesis that “eating more fruits and vegetables will improve your health” can be tested by gathering data on the health of people who eat different amounts of fruits and vegetables. If the people who eat more fruits and vegetables are healthier than those who eat less fruits and vegetables, then the hypothesis is supported.

Hypotheses are essential for scientific inquiry. They help scientists to focus their research, to design experiments, and to interpret their results. They are also essential for the development of scientific theories.

Types Of Hypothesis

In research, you typically encounter two types of hypothesis: the alternative hypothesis (which proposes a relationship between variables) and the null hypothesis (which suggests no relationship).

Simple Hypothesis

It illustrates the association between one dependent variable and one independent variable. For instance, if you consume more vegetables, you will lose weight more quickly. Here, increasing vegetable consumption is the independent variable, while weight loss is the dependent variable.

Complex Hypothesis

It exhibits the relationship between at least two dependent variables and at least two independent variables. Eating more vegetables and fruits results in weight loss, radiant skin, and a decreased risk of numerous diseases, including heart disease.

Directional Hypothesis

It shows that a researcher wants to reach a certain goal. The way the factors are related can also tell us about their nature. For example, four-year-old children who eat well over a time of five years have a higher IQ than children who don’t eat well. This shows what happened and how it happened.

Non-directional Hypothesis

When there is no theory involved, it is used. It is a statement that there is a connection between two variables, but it doesn’t say what that relationship is or which way it goes.

Null Hypothesis

It says something that goes against the theory. It’s a statement that says something is not true, and there is no link between the independent and dependent factors. “H 0 ” represents the null hypothesis.

Associative and Causal Hypothesis

When a change in one variable causes a change in the other variable, this is called the associative hypothesis . The causal hypothesis, on the other hand, says that there is a cause-and-effect relationship between two or more factors.

Examples Of Hypothesis

Examples of simple hypotheses:

Students who consume breakfast before taking a math test will have a better overall performance than students who do not consume breakfast.
Students who experience test anxiety before an English examination will get lower scores than students who do not experience test anxiety.
Motorists who talk on the phone while driving will be more likely to make errors on a driving course than those who do not talk on the phone, is a statement that suggests that drivers who talk on the phone while driving are more likely to make mistakes.

Examples of a complex hypothesis:

Individuals who consume a lot of sugar and don’t get much exercise are at an increased risk of developing depression.
Younger people who are routinely exposed to green, outdoor areas have better subjective well-being than older adults who have limited exposure to green spaces, according to a new study.
Increased levels of air pollution led to higher rates of respiratory illnesses, which in turn resulted in increased costs for healthcare for the affected communities.

Examples of Directional Hypothesis:

The crop yield will go up a lot if the amount of fertilizer is increased.
Patients who have surgery and are exposed to more stress will need more time to get better.
Increasing the frequency of brand advertising on social media will lead to a significant increase in brand awareness among the target audience.

Examples of Non-Directional Hypothesis (or Two-Tailed Hypothesis):

The test scores of two groups of students are very different from each other.
There is a link between gender and being happy at work.
There is a correlation between the amount of caffeine an individual consumes and the speed with which they react.

Examples of a null hypothesis:

Children who receive a new reading intervention will have scores that are different than students who do not receive the intervention.
The results of a memory recall test will not reveal any significant gap in performance between children and adults.
There is not a significant relationship between the number of hours spent playing video games and academic performance.

Examples of Associative Hypothesis:

There is a link between how many hours you spend studying and how well you do in school.
Drinking sugary drinks is bad for your health as a whole.
There is an association between socioeconomic status and access to quality healthcare services in urban neighborhoods.

Functions Of Hypothesis

The research issue can be understood better with the help of a hypothesis, which is why developing one is crucial. The following are some of the specific roles that a hypothesis plays: (Rashid, Apr 20, 2022)

A hypothesis gives a study a point of concentration. It enlightens us as to the specific characteristics of a study subject we need to look into.
It instructs us on what data to acquire as well as what data we should not collect, giving the study a focal point .
The development of a hypothesis improves objectivity since it enables the establishment of a focal point.
A hypothesis makes it possible for us to contribute to the development of the theory. Because of this, we are in a position to definitively determine what is true and what is untrue .

How will Hypothesis help in the Scientific Method?

The scientific method begins with observation and inquiry about the natural world when formulating research questions. Researchers can refine their observations and queries into specific, testable research questions with the aid of hypothesis. They provide an investigation with a focused starting point.
Hypothesis generate specific predictions regarding the expected outcomes of experiments or observations. These forecasts are founded on the researcher’s current knowledge of the subject. They elucidate what researchers anticipate observing if the hypothesis is true.
Hypothesis direct the design of experiments and data collection techniques. Researchers can use them to determine which variables to measure or manipulate, which data to obtain, and how to conduct systematic and controlled research.
Following the formulation of a hypothesis and the design of an experiment, researchers collect data through observation, measurement, or experimentation. The collected data is used to verify the hypothesis’s predictions.
Hypothesis establish the criteria for evaluating experiment results. The observed data are compared to the predictions generated by the hypothesis. This analysis helps determine whether empirical evidence supports or refutes the hypothesis.
The results of experiments or observations are used to derive conclusions regarding the hypothesis. If the data support the predictions, then the hypothesis is supported. If this is not the case, the hypothesis may be revised or rejected, leading to the formulation of new queries and hypothesis.
The scientific approach is iterative, resulting in new hypothesis and research issues from previous trials. This cycle of hypothesis generation, testing, and refining drives scientific progress.

Importance Of Hypothesis

Hypothesis are testable statements that enable scientists to determine if their predictions are accurate. This assessment is essential to the scientific method, which is based on empirical evidence.
Hypothesis serve as the foundation for designing experiments or data collection techniques. They can be used by researchers to develop protocols and procedures that will produce meaningful results.
Hypothesis hold scientists accountable for their assertions. They establish expectations for what the research should reveal and enable others to assess the validity of the findings.
Hypothesis aid in identifying the most important variables of a study. The variables can then be measured, manipulated, or analyzed to determine their relationships.
Hypothesis assist researchers in allocating their resources efficiently. They ensure that time, money, and effort are spent investigating specific concerns, as opposed to exploring random concepts.
Testing hypothesis contribute to the scientific body of knowledge. Whether or not a hypothesis is supported, the results contribute to our understanding of a phenomenon.
Hypothesis can result in the creation of theories. When supported by substantive evidence, hypothesis can serve as the foundation for larger theoretical frameworks that explain complex phenomena.
Beyond scientific research, hypothesis play a role in the solution of problems in a variety of domains. They enable professionals to make educated assumptions about the causes of problems and to devise solutions.

Research Hypotheses: Did you know that a hypothesis refers to an educated guess or prediction about the outcome of a research study?

It’s like a roadmap guiding researchers towards their destination of knowledge. Just like a compass points north, a well-crafted hypothesis points the way to valuable discoveries in the world of science and inquiry.

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Gregor Mendel’s Courage and Persistence

Mendel was curious about how traits were transferred from one generation to the next, so he set out to understand the principles of heredity in the mid-1860s. Peas were a good model system, because he could easily control their fertilization by transferring pollen with a small paintbrush. This pollen could come from the same flower (self-fertilization), or it could come from another plant's flowers (cross-fertilization). First, Mendel observed plant forms and their offspring for two years as they self-fertilized, or "selfed," and ensured that their outward, measurable characteristics remained constant in each generation. During this time, Mendel observed seven different characteristics in the pea plants, and each of these characteristics had two forms (Figure 3). The characteristics included height (tall or short), pod shape (inflated or constricted), seed shape (smooth or winkled), pea color (green or yellow), and so on. In the years Mendel spent letting the plants self, he verified the purity of his plants by confirming, for example, that tall plants had only tall children and grandchildren and so forth. Because the seven pea plant characteristics tracked by Mendel were consistent in generation after generation of self-fertilization, these parental lines of peas could be considered pure-breeders (or, in modern terminology, homozygous for the traits of interest). Mendel and his assistants eventually developed 22 varieties of pea plants with combinations of these consistent characteristics.

Mendel not only crossed pure-breeding parents, but he also crossed hybrid generations and crossed the hybrid progeny back to both parental lines. These crosses (which, in modern terminology, are referred to as F 1 , F 1 reciprocal, F 2 , B 1 , and B 2 ) are the classic crosses to generate genetically hybrid generations.

Understanding Dominant Traits

Understanding recessive traits.

When conducting his experiments, Mendel designated the two pure-breeding parental generations involved in a particular cross as P 1 and P 2 , and he then denoted the progeny resulting from the crossing as the filial, or F 1 , generation. Although the plants of the F 1 generation looked like one parent of the P generation, they were actually hybrids of two different parent plants. Upon observing the uniformity of the F 1 generation, Mendel wondered whether the F 1 generation could still possess the nondominant traits of the other parent in some hidden way.

To understand whether traits were hidden in the F 1 generation, Mendel returned to the method of self-fertilization. Here, he created an F 2 generation by letting an F 1 pea plant self-fertilize (F 1 x F 1 ). This way, he knew he was crossing two plants of the exact same genotype . This technique, which involves looking at a single trait, is today called a monohybrid cross . The resulting F 2 generation had seeds that were either round or wrinkled. Figure 4 shows an example of Mendel's data.

When looking at the figure, notice that for each F 1 plant, the self-fertilization resulted in more round than wrinkled seeds among the F 2 progeny. These results illustrate several important aspects of scientific data:

Multiple trials are necessary to see patterns in experimental data.
There is a lot of variation in the measurements of one experiment.
A large sample size, or "N," is required to make any quantitative comparisons or conclusions.

In Figure 4, the result of Experiment 1 shows that the single characteristic of seed shape was expressed in two different forms in the F 2 generation: either round or wrinkled. Also, when Mendel averaged the relative proportion of round and wrinkled seeds across all F 2 progeny sets, he found that round was consistently three times more frequent than wrinkled. This 3:1 proportion resulting from F 1 x F 1 crosses suggested there was a hidden recessive form of the trait. Mendel recognized that this recessive trait was carried down to the F 2 generation from the earlier P generation .

Mendel and Alleles

As mentioned, Mendel's data did not support the ideas about trait blending that were popular among the biologists of his time. As there were never any semi-wrinkled seeds or greenish-yellow seeds, for example, in the F 2 generation, Mendel concluded that blending should not be the expected outcome of parental trait combinations. Mendel instead hypothesized that each parent contributes some particulate matter to the offspring. He called this heritable substance "elementen." (Remember, in 1865, Mendel did not know about DNA or genes.) Indeed, for each of the traits he examined, Mendel focused on how the elementen that determined that trait was distributed among progeny. We now know that a single gene controls seed form, while another controls color, and so on, and that elementen is actually the assembly of physical genes located on chromosomes. Multiple forms of those genes, known as alleles , represent the different traits. For example, one allele results in round seeds, and another allele specifies wrinkled seeds.

One of the most impressive things about Mendel's thinking lies in the notation that he used to represent his data. Mendel's notation of a capital and a lowercase letter ( Aa ) for the hybrid genotype actually represented what we now know as the two alleles of one gene : A and a . Moreover, as previously mentioned, in all cases, Mendel saw approximately a 3:1 ratio of one phenotype to another. When one parent carried all the dominant traits ( AA ), the F 1 hybrids were "indistinguishable" from that parent. However, even though these F 1 plants had the same phenotype as the dominant P 1 parents, they possessed a hybrid genotype ( Aa ) that carried the potential to look like the recessive P 1 parent ( aa ). After observing this potential to express a trait without showing the phenotype, Mendel put forth his second principle of inheritance: the principle of segregation . According to this principle, the "particles" (or alleles as we now know them) that determine traits are separated into gametes during meiosis , and meiosis produces equal numbers of egg or sperm cells that contain each allele (Figure 5).

Dihybrid Crosses

Mendel had thus determined what happens when two plants that are hybrid for one trait are crossed with each other, but he also wanted to determine what happens when two plants that are each hybrid for two traits are crossed. Mendel therefore decided to examine the inheritance of two characteristics at once. Based on the concept of segregation , he predicted that traits must sort into gametes separately. By extrapolating from his earlier data, Mendel also predicted that the inheritance of one characteristic did not affect the inheritance of a different characteristic.

Mendel tested this idea of trait independence with more complex crosses. First, he generated plants that were purebred for two characteristics, such as seed color (yellow and green) and seed shape (round and wrinkled). These plants would serve as the P 1 generation for the experiment. In this case, Mendel crossed the plants with wrinkled and yellow seeds ( rrYY ) with plants with round, green seeds ( RRyy ). From his earlier monohybrid crosses, Mendel knew which traits were dominant: round and yellow. So, in the F 1 generation, he expected all round, yellow seeds from crossing these purebred varieties, and that is exactly what he observed. Mendel knew that each of the F 1 progeny were dihybrids; in other words, they contained both alleles for each characteristic ( RrYy ). He then crossed individual F 1 plants (with genotypes RrYy ) with one another. This is called a dihybrid cross . Mendel's results from this cross were as follows:

315 plants with round, yellow seeds
108 plants with round, green seeds
101 plants with wrinkled, yellow seeds
32 plants with wrinkled, green seeds

Thus, the various phenotypes were present in a 9:3:3:1 ratio (Figure 6).

Next, Mendel went through his data and examined each characteristic separately. He compared the total numbers of round versus wrinkled and yellow versus green peas, as shown in Tables 1 and 2.

Table 1: Data Regarding Seed Shape

315 + 108 = 423 101 + 32 = 133 3.2 1

Table 2: Data Regarding Pea Color

315 + 101 = 416 108 + 32 = 140 2.97 1

The proportion of each trait was still approximately 3:1 for both seed shape and seed color. In other words, the resulting seed shape and seed color looked as if they had come from two parallel monohybrid crosses; even though two characteristics were involved in one cross, these traits behaved as though they had segregated independently. From these data, Mendel developed the third principle of inheritance: the principle of independent assortment . According to this principle, alleles at one locus segregate into gametes independently of alleles at other loci. Such gametes are formed in equal frequencies.

Mendel’s Legacy

More lasting than the pea data Mendel presented in 1862 has been his methodical hypothesis testing and careful application of mathematical models to the study of biological inheritance. From his first experiments with monohybrid crosses, Mendel formed statistical predictions about trait inheritance that he could test with more complex experiments of dihybrid and even trihybrid crosses. This method of developing statistical expectations about inheritance data is one of the most significant contributions Mendel made to biology.

But do all organisms pass their on genes in the same way as the garden pea plant? The answer to that question is no, but many organisms do indeed show inheritance patterns similar to the seminal ones described by Mendel in the pea. In fact, the three principles of inheritance that Mendel laid out have had far greater impact than his original data from pea plant manipulations. To this day, scientists use Mendel's principles to explain the most basic phenomena of inheritance.

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What Are Genes, DNA, and Chromosomes?

The unique coding that determines an individual's inherited traits

What Is a Genome?

What is dna, what is a gene.

What Are Chromosomes?

What Is Genetic Variation?

Genetics is the study of heredity, meaning the traits that we inherit from our parents, they inherited from their parents, and so on. These traits are controlled by coded information found in every cell of the body.

This code is written in DNA, genes, and chromosomes. Together, these units make up the complete set of genetic instructions for every individual—referred to as a genome—including our sex, appearance, and medical conditions we may be at risk of. No two people have the same genome.

This article offers a basic explanation of genetics, including what genes, DNA, and chromosomes are. It also looks at errors in genetic coding that may place a person at risk of genetic diseases or birth defects.

In the simplest terms, a genome is the complete set of genetic instructions that determine the traits (characteristics and conditions) of an organism. It is made up of DNA, genes, and chromosomes.

DNA is a molecule in cells that carries the genetic information. It is made up of building blocks. The genetic coding of our traits is based on how these building blocks are arranged.

Genes are segments of DNA that determine our traits. Every human has between 20,000 and 25,000 different genes, half of which are inherited from our biological mothers and the other half from our biological fathers.

Chromosomes are long, bundled strands of DNA, each of which contains many genes. In total, there are two sets of 23 chromosomes in a cell. Each set is inherited from our biological parents.

Your genome determines how your body will develop before birth. It directs how you will grow, look, and age. And, it will determine how cells, tissues, and organs of the body work (including times when they may not work as they should).

While the genome of each species is distinct, every organism within that species has its own unique genome. This is why no two people are exactly alike, even twins.

In the simplest terms, DNA ( deoxyribonucleic acid ) is the building blocks of your genes.

Within DNA is a unique chemical code that guides your growth, development, and function. The code is determined by the arrangement of four chemical compounds known as nucleotide bases.

The four bases are:

Adenine (A)
Cytosine (C)
Guanine (G)
Thymine (T)

The bases pair up with each other—A with T and C with G—to form units known as base pairs. The pairs are then attached to form what ultimately looks like a spiraling ladder, known as a double helix .

The specific order, or sequence, of bases determines which instructions are given for building and maintaining an organism.

Human DNA consists of around 3 billion of these bases, 99% of which are exactly the same for all humans. The remaining 1% is what differentiates one human from the next.

Nearly every cell in a person’s body has the same DNA.

A gene is a unit of DNA that is encoded for a specific purpose.

Some genes provide instructions to produce particular proteins. Proteins are molecules that not only make up tissues like muscles and skin but also play many critical roles in the structure and function of the body.

Genes are encoded to produce RNA ( ribonucleic acid ), a molecule that converts the information stored in DNA to make the protein.

How genes are encoded will ultimately determine how you look and how your body works. Every person has two copies of each gene, one inherited from each parent.

Different versions of a gene are known as alleles . The alleles you inherit from your parents may determine, for example, if you have brown eyes or blue eyes. Other alleles may result in congenital (inherited) disorders like cystic fibrosis or Huntington’s disease , Other alleles may not cause disease but can increase your risk of getting things like cancer .

Genes only make up between 1% and 5% of the human genome. The rest is made up of non-coded DNA that doesn't produce protein but helps regulate how genes function.

What Is a Chromosome?

Genes are packaged into bundles known as chromosomes. Humans have 23 pairs of chromosomes for a total of 46 individual chromosomes. Chromosomes are contained within the control center (nucleus) of nearly every cell of the body.

One pair of chromosomes, called the sex chromosomes , determines whether you are born male or female. Females have a pair of XX chromosomes, while males have a pair of XY chromosomes.

The other 22 pairs, called autosomal chromosomes , determine the rest of your body’s makeup. Certain genes within these chromosomes may either be dominant or recessive.

By definition:

Autosomal dominant means that you need only one copy of an allele from one parent for a trait to develop (such as brown eyes or Huntington's disease).
Autosomal recessive means that you need two copies of the allele—one from each parent—for a trait to develop (such as blue eyes or cystic fibrosis).

Genes are prone to coding errors. Many errors won't make any significant difference in the structure or function of a person's body, but some can.

Some genetic variations will directly cause a defect or disease, some of which may be apparent at birth and others of which may only be seen later in life. Other variations can lead to changes in the gene pool that will affect inheritance patterns in later generations.

There are three common types of genetic variation:

Genetic Mutations

A genetic mutation is a change in the sequence of DNA. This is often due to copying errors that occur when a cell divides. It can also be caused by an infection, chemicals, or radiation that damages the structure of genes.

Genetic disorders like sickle cell disease , Tay-Sachs disease , and phenylketonuria are all caused by the mutation of a single gene. Radiation-induced cancer is caused by genetic changes caused by excessive exposure to medical or occupational radiation.

Genetic Recombination

Genetic recombination is a process in which pieces of DNA are broken, recombined, and repaired to produce a new allele. Also referred to as "genetic reshuffling," recombination occurs randomly in nature as a normal event during cell division. The new allele is then passed from parents to offspring.

Down syndrome is one such example of genetic recombination.

Genetic Migration

Genetic migration is an evolutionary process in which the addition or loss of people in a population changes the gene pool, making certain traits either less common or more common.

A theoretical example is the loss of red-haired people from Scotland, which over time may result in fewer and fewer Scottish children being born with red hair.

DNA is the building blocks of genes that contain the coded instruction for building and maintaining a body. Genes are a portion of DNA that are tasked with making specific proteins that play a critical role in the structure and function of the body. Chromosomes are structures containing many genes each. They are passed from parents to offspring and determine an individual's unique traits.

Together, DNA, genes, and chromosomes make up each organism's genome. Every organism—and every individual—has a unique genome.

A Word From Verywell

Genetics increasingly informs the way in which diseases are diagnosed, treated, or prevented. Many of the tools used in medicine today were the result of a greater understanding of DNA, genes, chromosomes, and the human genome as a whole.

Today, genetic research has led to the development of targeted drugs that can treat cancer with less damage to non-cancerous cells. Genetic tests are available to predict your likelihood of certain diseases.

Genetic engineering has even allowed scientists to mass-produce human insulin in bacteria and create RNA vaccines like some of those used to treat COVID-19 .

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By Mary Kugler, RN Mary Kugler, RN, is a pediatric nurse whose specialty is caring for children with long-term or severe medical problems.

Reviewed by Psychology Today Staff

Genetics is the study of genes and the variation of characteristics that are influenced by genes—including physical and psychological characteristics. All human traits, from one's height to one's fear of heights , are driven by a complex interplay between the expression of inherited genes and feedback from the environment .

Scientists are tasked with a massive but increasingly plausible mission: mapping the pathway from one's genes to the person one sees in the mirror. What they learn about the power of genes has implications for understanding mental illness and psychological differences between individuals, as well as the psychological effects of non-genetic factors.

Why Genes Matter in Psychology
The Science of Genetics and Behavior

Genes help to define who an individual is inside and out. While non-genetic factors have a role to play, too, what scientists have learned about these influences can clash with common wisdom . A characteristic or behavior that appears to result from a child’s upbringing—such as a proneness to mental illness or divorce — may actually be largely a product of the genes she inherited from her parents. In fact, research investigating the influence of the family environment suggests that it accounts for a surprisingly small amount of the difference between people on characteristics that scientists measure.

A gene is the basic unit through which genetic information is stored and passed between generations. Physically, a gene is a specific section of one of the long, double-helix-shaped DNA molecules that appear in each cell of the body. Genes vary in size, comprising anywhere from hundreds to millions of the nucleotides that collectively make up DNA. Many (but not all) genes provide chemical “instructions” for the creation of protein molecules, or serve other roles that are integral to the function of an organism. Different versions of the same gene are called alleles.

The genome is the entirety of the genetic material contained in an individual. Human DNA is estimated to contain between 20,000 and 25,000 genes. The vast majority of each person’s genome is identical to that of the next person, but the portion that differs is consequential for how individuals develop.

A chromosome is a structure within a cell nucleus that is made up of a long DNA molecule and proteins that provide support. Each human cell contains 23 pairs of chromosomes, which together store a person’s genetic code.

In addition to visible traits like weight and eye color, psychological qualities such as personality traits (such as extraversion and agreeableness ), intelligence , risk of mental illness, and many others are to some extent influenced by genetics. While genes do not account for all of the differences between people on these characteristics, research indicates that they have a substantial impact.

People’s levels of risk for all major psychiatric disorders are determined partly by genetics. Estimates of how much variability in risk can be attributed to genetic differences between individuals include: 75 percent or more for ADHD , autism spectrum disorders, bipolar disorder , and schizophrenia; 50 to 60 percent for alcohol dependence and anorexia nervosa, and 20-45 percent for anxiety disorders, OCD , PTSD , and major depressive disorder.

The pathway from genes to psychological dispositions and behavior is highly complex, but it runs through the brain: Genetic instructions influence brain development, and differences in the genetic code produce differences in how the brain is wired. These instructions do not completely determine how development plays out, leaving room for other factors ( including chance events during development ) in shaping behavior.

Genes may also influence a person less directly, through chains of cause-and-effect that involve the environment . For example, a genetically influenced trait (such as above-average extraversion) might lead someone to seek out situations (such as frequent social interactions) that reinforce that trait.

The phrase “nature vs. nurture” has been used as a shorthand for the question of how much a people’s inherent nature, or genetics, explain the differences between them, how much of these differences are instead explained by “nurture”—meaning upbringing, or people’s experience of their environments more generally.

While both genetics and environment play a part in shaping people’s characteristics, the phrase “nature vs. nurture” can be misleading. In many ways, genetics and experience interact (rather than working in opposition) to affect how a person turns out. And some portion of individual differences are due to neither nature nor nurture alone, but rather to inherent variability in the process of human development.

Chemical compounds can attach to genes, modifying their activity without changing the underlying DNA. These modifications are called epigenetic changes (and the study of them is epigenetics ). Epigenetic changes happen normally as part of development, but they can also be influenced by environmental and experiential factors, such as diet and exposure to toxins. There is evidence suggesting that traumatic experiences may also lead to epigenetic changes that influence gene expression.

There is no one gene that activates a particular psychiatric disorder or any other complex psychological trait. In fact, many genes interact to influence the human brain. Normal and disordered psychological characteristics are polygenic , meaning that they are each shaped by a large number of genes. While rare mutations in certain genes may have a disproportionate impact, for the most part, each of the many relevant genetic differences plays a very small role in increasing or decreasing risk of a particular condition or influencing a given trait.

In medicine more broadly, there are certain genetic disorders that do primarily involve abnormalities in a single gene. These single-gene disorders include cystic fibrosis, Huntington disease, sickle-cell anemia, and Duchenne muscular dystrophy.

The X and Y chromosomes are also known as the sex chromosomes and play a fundamental role in determining the biological sex characteristics of an individual (such as reproductive organs). Females do not have Y chromosomes; they inherit one X chromosome from each parent. Males have an X chromosome, form their mother, and a Y chromosome from their father. The X and Y chromosomes also contain genes that affect traits not related to an individual’s sex.

Genealogy is the study of family lineage. While it is not a new practice, developments in DNA testing in recent decades brought a boom in enthusiasm about genealogy, with businesses using test results to provide clues about consumers’ genetic relatives and their geographic origins. There are a variety of reasons people might be drawn to genealogy , such as curiosity about a family’s deeper history among those who feel that they lack knowledge of or cultural ties to the past. One of the broader lessons of genealogy may be that far-flung people are more related than they might seem.

It may seem obvious that the genes people inherit from their parents and share with their siblings have an effect on behavior and temperament. Individuals are often noticeably more similar in a variety of ways to immediate family members than they are to more distant ones, or to non-relatives. Of course, there are plenty of notable differences within families as well. Scientists have employed an array of methods to drill down into how and to what extent genetic differences truly account for psychological differences.

Behavioral genetics, or behavior genetics, is the study of psychological differences between individuals and how genetic and non-genetic factors create those differences. Among other questions, behavioral genetics researchers have sought to determine the extent to which various specific differences in people’s behaviors and traits can be explained by differences in their genetic code.

Scientists use specialized methods to explore the links between genes and individual differences. Studies of twins who either do or do not have identical genomes allow for estimates of the degree to which genes drive the variation in psychological traits. Other methods, such as studying adopted children and their adoptive and biological parents, have been used as well. In recent years, genome-wide association studies (GWAS) have emerged as a major approach in behavioral genetics. A GWAS uses genetic testing to identify numerous genetic differences across many individuals , then analyze the association between these differences and personality traits or other outcomes.

Heritability is a measure of how much of the differences between people on a given characteristic can be attributed to genetics. More specifically, it is an estimate of the amount of variation between individuals in a given population that can be accounted for by genetic differences. Behavioral genetics research indicates that every trait is (at least) a little bit heritable —though the fact that a trait is heritable does not mean it is fixed.

Heritability estimates range from 0 to 1, or from zero percent to 100 percent. For example, if the heritability of a trait is estimated to be 50 percent, that suggests that about half of the overall variation between different people on measures of that trait—within the specific group of people measured—is due to differences in their DNA. (If heritability of a trait was 100 percent, identical twins , who share the same genetic code, would be exactly the same on that trait—but that doesn’t actually happen.)

Genes and the environment do not work completely independently. In what scientists call gene-environment interaction, aspects of the environment may have different effects on an individual depending on her genetic code. For example, adverse childhood experiences may have a severe impact on someone with a certain genetic disposition and a less-severe effect on someone with different genes. Gene-environment interaction can also work the other way around: the influence of genes on an outcome may depend on a person’s environment.

Given that genetics only accounts for a portion of the psychological differences between people, a genetic test will never be able to perfectly predict an individual’s behavior. Some researchers are hopeful that “polygenic scores”—which provide information about the likelihood of certain outcomes (such as developing a mental disorder) based on many small genetic variations —will serve as a useful tool for assessing risk in psychological domains. However, the predictive power of such scores is currently limited, and there are further limitations that lead other researchers to question how effective they will ultimately be for forecasting psychological outcomes.

Genome editing is a rapidly developing practice through which an organism’s genetic code is deliberately changed. Many of the genome-editing approaches in development today involve a mechanism called CRISPR/Cas9, which is adapted from a genome-editing system that appears in bacteria (as a defense against viruses). CRISPR enables the relatively quick and efficient targeting of specific genes in a cell. Scientists are pursuing CRISPR-based therapies for potential use in humans, with aims such as removing disease-causing genetic mutations. Genome editing technology could ultimately play a role in the treatment of mental illness.

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Wobble Hypothesis (With Diagram) | Genetics

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In this article we will discuss about the concept of wobble hypothesis.

Crick (1966) proposed the ‘wobble hypothesis’ to explain the degeneracy of the genetic code. Except for tryptophan and methionine, more than one codons direct the synthesis of one amino acid. There are 61 codons that synthesise amino acids, therefore, there must be 61 tRNAs each having different anticodons. But the total number of tRNAs is less than 61.

This may be explained that the anticodons of some tRNA read more than one codon. In addition, identity of the third codon seems to be unimportant. For example CGU, CGC, CGA and CGG all code for arginine. It appears that CG specifies arginine and the third letter is not important. Conventionally, the codons are written from 5′ end to 3′ end.

Therefore, the first and second bases specify amino acids in some cases. According to the Wobble hypothesis, only the first and second bases of the triple codon on 5′ → ‘3 mRNA pair with the bases of the anticodon of tRNA i.e A with U, or G with C.

The pairing of the third base varies according to the base at this position, for example G may pair with U. The conventional pairing (A = U, G = C) is known as Watson-Crick pairing (Fig. 7.1) and the second abnormal pairing is called wobble pairing.

This was observed from the discovery that the anticodon of yeast alanine-tRNA contains the nucleoside inosine (a deamination product of adenosine) in the first position (5′ → 3′) that paired with the third base of the codon (5′ → 3′). Inosine was also found at the first position in other tRNAs e.g. isoleucine and serine.

The purine, inosine, is a wobble nucleotide and is similar to guanine which normally pairs with A, U and C. For example a glycine-tRNA with anticodon 5′-ICC-3′ will pair with glycine codons GGU, GGC, GGA and GGG (Fig 7.2). Similarly, a seryl-tRNA with anticodon 5′-IGA-3′ pairs with serine codons UCC, UCU and UCA (5-3′). The U at the wobble position will be able to pair with an adenine or a guanine.

According to Wobble hypothesis, allowed base pairings are given in Table 7.5:

Due to the Wobble base pairing one tRNA becomes able to recognise more than one codons for an individual amino acid. By direct sequence of several tRNA molecules, the wobble hypothesis is confirmed which explains the pattern of redundancy in genetic code in some anticodons (e.g. the anticodons containing U, I and G in the first position in 5’→ 3′ direction)

The seryl-tRNA anticodon (UCG) 5′-GCU-3′ base pairs with two serine codons, 5′-AGC-3′ and 5′-AGU-3′. Generally, Watson-Crick pairing occurs between AGC and GCU. However, in AGU and GCU pairing, hydrogen bonds are formed between G and U. Such abnormal pairing called ‘Wobble pairing’ is given in Table 7.5.

Three types of wobble pairings have been proposed:

(i) U in the wobble position of the tRNA anticodon pairs with A or G of codon,

(ii) G pairs with U or C, and

(iii) 1 pairs with A, U or C.

Short Notes on Anticodons | Genetics
Genetic Code: Degeneracy and Universality | Protein

Microbiology , Genetics , Wobble Hypothesis , Concept of Wobble Hypothesis

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Definition of hypothesis noun from the Oxford Advanced Learner's Dictionary

to formulate/confirm a hypothesis
a hypothesis about the function of dreams
There is little evidence to support these hypotheses.
formulate/advance a theory/hypothesis
build/construct/create/develop a simple/theoretical/mathematical model
develop/establish/provide/use a theoretical/conceptual framework
advance/argue/develop the thesis that…
explore an idea/a concept/a hypothesis
make a prediction/an inference
base a prediction/your calculations on something
investigate/evaluate/accept/challenge/reject a theory/hypothesis/model
design an experiment/a questionnaire/a study/a test
do research/an experiment/an analysis
make observations/measurements/calculations
carry out/conduct/perform an experiment/a test/a longitudinal study/observations/clinical trials
run an experiment/a simulation/clinical trials
repeat an experiment/a test/an analysis
replicate a study/the results/the findings
observe/study/examine/investigate/assess a pattern/a process/a behaviour
fund/support the research/project/study
seek/provide/get/secure funding for research
collect/gather/extract data/information
yield data/evidence/similar findings/the same results
analyse/examine the data/soil samples/a specimen
consider/compare/interpret the results/findings
fit the data/model
confirm/support/verify a prediction/a hypothesis/the results/the findings
prove a conjecture/hypothesis/theorem
draw/make/reach the same conclusions
read/review the records/literature
describe/report an experiment/a study
present/publish/summarize the results/findings
present/publish/read/review/cite a paper in a scientific journal
Her hypothesis concerns the role of electromagnetic radiation.
Her study is based on the hypothesis that language simplification is possible.
It is possible to make a hypothesis on the basis of this graph.
None of the hypotheses can be rejected at this stage.
Scientists have proposed a bold hypothesis.
She used this data to test her hypothesis
The hypothesis predicts that children will perform better on task A than on task B.
The results confirmed his hypothesis on the use of modal verbs.
These observations appear to support our working hypothesis.
a speculative hypothesis concerning the nature of matter
an interesting hypothesis about the development of language
Advances in genetics seem to confirm these hypotheses.
His hypothesis about what dreams mean provoked a lot of debate.
Research supports the hypothesis that language skills are centred in the left side of the brain.
The survey will be used to test the hypothesis that people who work outside the home are fitter and happier.
This economic model is really a working hypothesis.
speculative
concern something
be based on something
predict something
on a/the hypothesis
hypothesis about
hypothesis concerning

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Article Contents

The classical period of genetics, the neoclassical period of genetics, the breakdown of the neoclassical concept of the gene and the beginning of the modern period of genetics, current status and future perspectives regarding the concept of the gene, putting it all together: toward a new definition of the “gene”, acknowledgments, literature cited, the evolving definition of the term “gene”.

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Petter Portin, Adam Wilkins, The Evolving Definition of the Term “Gene”, Genetics , Volume 205, Issue 4, 1 April 2017, Pages 1353–1364, https://doi.org/10.1534/genetics.116.196956

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This paper presents a history of the changing meanings of the term “gene,” over more than a century, and a discussion of why this word, so crucial to genetics, needs redefinition today. In this account, the first two phases of 20th century genetics are designated the “classical” and the “neoclassical” periods, and the current molecular-genetic era the “modern period.” While the first two stages generated increasing clarity about the nature of the gene, the present period features complexity and confusion. Initially, the term “gene” was coined to denote an abstract “unit of inheritance,” to which no specific material attributes were assigned. As the classical and neoclassical periods unfolded, the term became more concrete, first as a dimensionless point on a chromosome, then as a linear segment within a chromosome, and finally as a linear segment in the DNA molecule that encodes a polypeptide chain. This last definition, from the early 1960s, remains the one employed today, but developments since the 1970s have undermined its generality. Indeed, they raise questions about both the utility of the concept of a basic “unit of inheritance” and the long implicit belief that genes are autonomous agents. Here, we review findings that have made the classic molecular definition obsolete and propose a new one based on contemporary knowledge.

IN 1866, Gregor Mendel, a Moravian scientist and Augustinian friar, working in what is today the Czech Republic, laid the foundations of modern genetics with his landmark studies of heredity in the garden pea ( Pisum sativum ) ( Mendel 1866 ). Though he did not speak of “genes”—a term that first appeared decades later—but rather of elements , and even “cell elements” (original German Zellelemente p. 42), it is clear that Mendel was hypothesizing the hereditary behavior of miniscule hidden factors or determinants underlying the stably inherited visible characteristics of an organism, which today we would call genes. This is apparent throughout his publication in his use of abstract letter symbols for hereditary determinants to denote the physical factors underlying the inheritance of characteristics. There is no doubt that he considered the mediators of heredity to be material entities, though he made no conjectures about their nature.

The word “gene” was not coined until early in the 20th century, by the Danish botanist Johannsen (1909) , but it rapidly became fundamental to the then new science of genetics, and eventually to all of biology. Its meaning, however, has been evolving since its birth. In the beginning, the concept was used as a mere abstraction. Indeed, Johannsen thought of the gene as some form of calculating element (a point to which we will return), but deliberately refrained from speculating about its physical attributes ( Johannsen 1909 ). By the second decade of the 20th century, however, a number of genes had been localized to specific positions on specific chromosomes, and could, at least, be treated, if not thought of precisely, as dimensionless points on chromosomes. Furthermore, groups of genes that showed some degree of coinheritance could be placed in “linkage groups,” which were the epistemic equivalent of the cytological chromosome. We term this phase the “classical period” of genetics. By the early 1940s, certain genes had been shown to have internal structure, and to be dissectable by genetic recombination; thus, the gene, at this point, had conceptually acquired a single dimension, length. Twenty years later, by the early 1960s, the gene had achieved what seemed like a definitive physical identity as a discrete sequence on a DNA molecule that encodes a polypeptide chain. At this point, the gene had a visualizable three-dimensional structure as a particular kind of molecule. We will call this period—from roughly the end of the 1930s to the early 1960s—the “neoclassical period.”

The 1960s definition of the gene is the one most geneticists employ today, but it is clearly out-of-date for deoxyribonucleic acid (DNA)-based organisms. (We will deal only with the latter; RNA viruses and their genes will not be discussed.) Here, we review the older history of the terminology, and then the findings from the 1970s onwards that have undermined the generality of the 1960s definition. We will then propose a contemporary definition of the “gene” that accounts for the complexities revealed in recent decades. This publication is a follow-on paper to an earlier paper by one of us ( Portin 2015 ).

The development of modern genetics began in 1900, when three botanists—the German Carl Correns, the Dutchman Hugo de Vries, and the Austrian Erich von Tschermak—independently cited and discussed the experiments of Mendel as basic to understanding the nature of heredity. They presented results similar to Mendel’s though using different plants as experimental material ( Correns 1900 ; de Vries 1900 ; Tschermak 1900 ). Their conceptual contributions as “rediscoverers” of Mendel, however, were probably not equivalent. De Vries and Correns claimed that they had discovered the essential facts and developed their interpretation before they found Mendel’s article, and they demonstrated that they fully understood the essential aspects of Mendel’s theory ( Stern 1970 ). In contrast, Tschermak’s analysis of his own data was inadequate, and his paper lacked an interpretation. Thus, while he sensed the significance of Mendel’s work, Tschermak should not be given credit equal to that due to de Vries and Correns.

In 1900, chromosomes were already known, and they were soon seen to provide a concrete basis for Mendel’s abstract hereditary factors. This postulated connection between genes and chromosomes, which later came to be known as the chromosome theory of inheritance, was initially provided by the German biologist T. H. Boveri and the American geneticist and physician W. S. Sutton during the years 1902–1903. Boveri first demonstrated the individuality of chromosomes with microscopic observations on the sea urchin Paracentrotus lividus ( Boveri 1902 ). He went on to demonstrate the continuity of chromosomes through cell divisions with studies of Ascaris megalocephala , a parasitic nematode worm ( Boveri 1903 ). These two characteristics—individuality and continuity—are necessary, although not sufficient, characteristics of the genetic material. Sutton’s contribution ( Sutton 1903 ), on the basis of his studies on the spermatogenesis of Brachystola magna , a large grasshopper, was to demonstrate a clear equivalence between the behavior of chromosomes at the meiotic divisions and Mendel’s postulated separation and independent inheritance of character differences at gamete formation. Thus, this early version of the chromosome theory of inheritance suggests an explanation for Mendel’s laws of inheritance: the law of segregation and the law of independent assortment. It was not until 1916, however, that it could be considered to be proven. In that year, C. B. Bridges, an American geneticist, showed in Drosophila melanogaster that nondisjunction, a rare exceptional behavior of genetic makers (lack of segregation) during gamete formation, was always associated with an analogous exceptional behavior of a given chromosome pair during meiosis ( Bridges 1916 ).

Shortly after the birth of the chromosome theory, however, a new phenomenon had been discovered that appeared to contradict Mendel’s law of independent assortment. This was the phenomenon of linkage, initially found in the sweet pea ( Lathyrus odoratus ), in which some genes were found to exhibit “coupling,” violating independent assortment ( Bateson et al. 1905a , b ). This exception to the rule, however, became the basis of an essential extension of the chromosome theory when it was realized that genes showing linkage are located on the same chromosome, and genes showing independent assortment are located on different chromosomes.

According to the canonical history of genetics, it was the American geneticist T. H. Morgan who was the first to propose in 1910 this extension of the chromosome theory ( Morgan 1910 , 1917 ). Recent studies on the history of genetics ( Edwards 2013 ), however, show that, most likely, Morgan was influenced by the first textbook of genetics in English written by R. H. Lock, a British botanist associated with Bateson and Punnet, published in 1906, where the possibility that linkage might result from genes lying on the same chromosome was first suggested ( Lock 1906 ). Thus, it is Lock to whom the credit of explaining linkage must be given.

It was soon understood that genes sufficiently far apart on the chromosome can also show independent assortment, due to extensive genetic recombination during meiosis, while genes that are closer to each other show a degree of coinheritance, the frequency of their separation by recombination being directly related to the distance between them. Owing to the work of Morgan and his group on the fruit fly ( D. melanogaster ), the phenomenon of linkage and its breakdown via crossing over became the essential basis for the mapping of genes ( Morgan 1919 , 1926 ; Morgan et al. 1915 ). The first map, of the Drosophila X-chromosome, was constructed by Alfred Sturtevant, one of Morgan’s students ( Sturtevant 1913 ). The linear sequence of genes he diagrammed was the abstract genetic epistemic equivalent of the chromosome itself.

The genetic maps of the linkage groups were subsequently followed by cytological maps of the chromosomes. These were first constructed by showing that X-ray-induced changes of the order of the genes in the linkage groups, such as translocations and deletions, were associated with corresponding changes in the structure of chromosomes ( Dobzhansky 1929 ; Muller and Painter 1929 ; Painter and Muller 1929 ). This was followed by detailed cytological mapping of genes, made possible by the existence of the “giant” chromosomes of the salivary glands of the fruit fly, in which genes identified by their inheritance patterns could be localized to specific (visible) locations on chromosomes ( Painter 1934 ; Bridges 1935 , 1938 ).

Morgan conceived the cytological explanation for the genetical phenomenon of crossing over by adopting the chiasmatype theory of Frans Alfons Jannsens, a Belgian cytologist, that was based on his observations of meiosis at spermatogenesis in the salamander Batrachoseps attenuatis ( Janssens 1909 ; see also Koszul et al. 2012 ). Janssens observed cross figures at synapsis in meiotic chromosome preparations of this amphibian that resembled the Greek letter chi (χ). Accordingly, he called such a junction “chiasma” (pl. chiasmata). Janssens interpreted each of these as due to fusion at one point between two of the four strands of the tetrad of chromatids at the pachytene stage of the meiotic prophase. According to the chiasmatype theory, chiasmata were due to breakage and reunion of one maternal and one paternal chromatid of the tetrad. Consequently, the formation of each chiasma leads to an exchange of equal and corresponding regions of two of the four chromatids. This mechanism of exchange provided the needed physical explanation for the partial genetic linkage of genes that Morgan had observed. In other words, chiasmata are cytological counterparts of the genetical crossover points.

An alternative explanation for the origin of chiasmata was the so called classical hypothesis, which did not require breakage and rejoining of chromosomes, but assumed that chiasmata were simply a result of the paternal and maternal chromatids going across each other, forming a cross-like configuration at the pachytene stage of meiosis ( McClung 1927 ; Sax 1932a , b ). This hypothesis did not explain the phenomenon of genetic recombination, but was preferred by most cytologists of that time because it did not threaten the permanence and individuality of the chromosomes, which the chiasmatype theory initially seemed to do. During subsequent years, many cytological facts, as reviewed, for example, in Whitehouse (1973) , supported the chiasmatype theory, but not the classical theory.

Thus, by the early 1930s, the concept of the gene had become more concrete. Genes were regarded as indivisible units of inheritance, each located at a specific point on a specific chromosome. Furthermore, they could be defined in terms of their behavior as fundamental units on the basis of four criteria: (1) hereditary transmission, (2) genetic recombination, (3) mutation, and (4) gene function. Moreover, it was believed, albeit without any empirical evidence, that these four ways of defining the gene fully agreed with one another (reviewed in Portin 1993 ; Keller 2000 ). As A. Sturtevant and G. W. Beadle wrote in (1939), near the end of what we are calling the classical period of genetics, it was also clear that genes determine the nature of developmental reactions and thus, ultimately, the visible traits they generate. But how genes do these things was unknown; indeed, that was considered one of the major unsolved problems in biology, and it remained so for two decades ( Sturtevant and Beadle 1962 , p. 335). Further, it was believed that the integration of genetics with such fields as biochemistry, developmental physiology, and experimental embryology would lead to a deep understanding of the nature and role of genes, and that this integration would add to our understanding of those processes that make up development ( Sturtevant and Beadle 1962 , p. 357; see also Sturtevant 1965 ).

The significance of this perspective was initially elaborated by H. J. Muller, an American geneticist and a student of Morgan’s, who had done important work on several key aspects of the subject: the mapping of genes ( Muller 1920 ), the relation between genes and characteristics of organisms ( Muller 1922 ), and the nature of gene mutation ( Muller 1927 ; also see Carlson 1966 ). In his classic paper dealing with the effect of changes in individual genes on the variation of the organism, Muller (1922 ) published arguments that can be viewed as a theoretical summary of the essence of the classical period of genetics. On the basis of a considerable body of earlier work, he put forward an influential theory that genes are molecules with three essential capacities: autocatalysis (self-reproduction), heterocatalysis (production of nongenetic material or effects), and ability to mutate (while retaining the first two properties). In this view, genes were undoubted physical entities, three-dimensional ultramicroscopic ones, possessing individuated heritable structures, with some capacity for change that itself could be passed on.

In another visionary paper, Muller (1926) connected the concept of the gene to the theory of evolution, while he described the gene as the basis of evolution and the origin of life itself, indeed as the basis of life itself. These profound views of Muller strongly influenced the direction of much future research, not only in genetics, but in biology as a whole ( Carlson 1966 p. 82).

Whatever the speculations of Muller and a few others, the classical period of genetics was one in which the gene could be treated effectively as a dimensionless point on a chromosome. It was followed, however, by what we are calling the neoclassical period, in which the gene first acquired an unambiguous spatial dimension, namely length, and later a likewise linear chemical identity, in the form of the DNA molecule. This period of genetics involved two different, but complementary, research programs: on the one hand, it was demonstrated, using the classic genetic tool of recombinational mapping, that genes have an internal structure; on the other hand, the basic molecular nature of the gene and its function began to be revealed. These two streams fused in the late 1950s.

The neoclassical period began in the early 1940s, with work in formal genetics showing that genes could be dissected into contiguous segments by genetic recombination. Hence, they were not dimensionless points but entities with length. These observations were made first in D. melanogaster ( Oliver 1940 ; Lewis 1941 ; 1945 ; Green and Green 1949 ), and then in microbial fungi ( Bonner 1950 ; Giles 1952 ; Pontecorvo 1952 ; Pritchard 1955 ).

If genes had length, however, they must be long molecules of some sort, and the question was whether those molecules were proteins or DNA, the two major molecular constituents of the chromosomes. Critically important work in the early 1940s, in the laboratory of Oswald Avery at Rockefeller University, answered the question. Avery and his colleagues showed that DNA is the hereditary material by demonstrating that the causative agent in bacterial transformation, which entailed a heritable change in the morphology of the bacterial cells ( Griffith 1928 ), was DNA ( Avery et al. 1944 ). Though this work was published in 1944, it would take nearly a decade for this to become universally accepted. The experimental proof that convinced the scientific community was the experiment of Hershey and Chase (1952) , in which these authors showed that the DNA component of bacteriophages was the one responsible for their multiplication.

The most critical and final breakthrough for the DNA theory of inheritance, however, was the revelation of the double-helical structure of DNA ( Watson and Crick 1953a , 1954 ), and the realization of the genetic implications of that ( Watson and Crick 1953b ). This was followed by demonstrations in the early 1960s that genes are first transcribed into messenger RNA (mRNA), which transmitted the genetic information from the nucleus to the protein synthesis machinery in the cytoplasm (reviewed in Portin 1993 ; Judson 1996 ). Earlier work in the 1940s had established the connection between genes and proteins, in the “one gene-one enzyme” hypothesis of Beadle and Tatum (1941) (see also Srb and Horowitz 1944 ; reviewed in Strauss 2016 ). By the late 1950s, there was thus a satisfying molecular theory of both the nature of the gene, and the connections between genes and proteins.

Crucial further work involved the genetic fine structure mapping of genes—a research program that reached its culmination with work by S. Benzer and C. Yanofsky. Benzer, using the operational cis -trans test, originated by E. B. Lewis in Drosophila , defined the unit of genetic complementation, i.e. , the basic unit of gene function, which he called the cistron ( Box 1 ). He also defined the smallest units of genetic recombination and gene mutation: the recon and muton, respectively ( Benzer 1955 , 1959 , 1961 ). The postulate of the classical period that the gene was a fundamental unit not only of function, but also of recombination and mutation, was definitively disproved by Benzer’s work showing that the “gene” had many mutons and recons. Yanofsky and his coworkers validated the material counterparts of these formal concepts of Benzer. The equivalent of the cistron is a sequence of nucleotide pairs in DNA that contain information for the synthesis of a polypeptide, and determines its amino acid sequences, an idea known as the colinearity hypothesis. Furthermore, the physical DNA equivalent of the recon and the muton was shown to be one nucleotide pair ( Crawford and Yanofsky 1958 ; Yanofsky and Crawford 1959 ; Yanofsky et al. 1964 , 1967 ). The period of neoclassical genetics culminated in the cracking of the universal genetic code by several teams, revealing that nucleotide sequences specify the sequence of polypeptide chains (reviewed in Ycas 1969 ; Judson 1996 ).

The neoclassical concept of the gene, outlined above, can be summarized in the formulation “one gene—one mRNA—one polypeptide,” which combines the idea of mRNA, as developed by Jacob and Monod (1961a) ; Gros et al. (1961) ; Brenner et al. (1961) , and the earlier “one gene—one enzyme” hypothesis of Beadle and Tatum (1941) (and see Srb and Horowitz 1944 ). Another version of this hypothesis is that of “one cistron—one polypeptide” ( Crick 1963 ), which emerged as a slogan in the 1960s–1980s. Altogether, the conceptual journey from Johannsen’s totally abstract entities termed “genes” to a defined, molecular idea of what a gene is, and how it works, had taken a little over half a century.

Deviations from the one gene—one mRNA—one polypeptide hypothesis

The hypothesis of “one gene—one mRNA—one polypeptide” as a general description of the gene and how it works started to expire, however, when it was realized that a single gene could produce more than one mRNA, and that one gene can be a part of several transcription units. This one-to-several relationship of genes to mRNAs occurs by means of complex promoters and/or alternative splicing of the primary transcript.

Multiple transcription initiation sites, i.e. , alternative promoters, have been found in all kingdoms of organisms, and they have been classified into six classes ( Schibler and Sierra 1987 ). All of them can produce transcripts that do not obey the rule of one-to-one correspondence between the gene and the transcription unit, since transcription can be initiated at different promoters. The result is that a single gene can produce more than one kind of transcript ( Schibler and Sierra 1987 ).

The discovery of alternative splicing as a way of producing different transcripts from one gene had a more complex history. In the late 1970s it was discovered, first in animal viruses and then in eukaryotes, that genes have a split structure. That is, genes are interrupted by introns (see review by Portin 1993 ). Split genes produce one pre-mRNA molecule, from which the introns are removed during the maturation of the mRNA by pre-mRNA splicing. Depending on the gene, the splicing pattern can be invariant (“constitutive”) or variable (“alternative”). In constitutive splicing, all the exons present in a transcript are incorporated into one mature mRNA through invariant ligation of consecutive exons, yielding a single kind of mRNA from the gene. In alternative splicing, nonconsecutive exons are joined by the processing of some, but not all, transcripts from a gene. In other words, individual exons can be excluded from the mature mRNA in some transcripts, but they can be included in others ( Leff et al. 1986 ; Black 2003 ). Alternative splicing is a regulated process, being tissue-specific and developmental-stage-specific. Nevertheless, the colinearity of the gene and the mRNA is preserved, since the order of the exons in the gene is not changed.

In addition to alternative splicing, two other phenomena are now known that contradict a basic tenet of the neoclassical gene concept, namely that amino-acid sequences of proteins, and consequently their functions, are always derivable from the DNA of the corresponding gene. These are the phenomena of RNA editing (reviewed by Brennickle et al. 1999 ; Witzany 2011 ) and of gene sharing originally found by J. Piatigorsky (reviewed in Piatigorsky 2007 ). The term RNA editing describes post-transcriptional molecular processes in which the structure of an RNA molecule is altered. Though a rare event, it has been observed to occur in eukaryotes, their viruses, archaea and prokaryotes, and involves several kinds of base modifications in RNA molecules. RNA editing in mRNAs effectively alters the amino acid sequence of the encoded protein so that it differs from that predicted by the genomic DNA sequence ( Brennickle et.al . 1999 ). The concept of gene sharing describes the fact that different cells contain identically sequenced polypeptides, derived from the same gene, but so differently configured in different cellular contexts that they perform wildly different functions. This phenomenon, facetiously called “protein moonlighting,” means that a gene may acquire and maintain a second function without gene duplication, and without loss of the primary function. Such genes are under two or more entirely different selective constraints ( Piatigorsky and Wistow 1989 ).

Despite these observations, showing the potential one-to-many relationships of genes to mRNAs and their encoded proteins, the concept of the gene remained intact; the gene itself could still be seen as a defined and localized nucleotide sequence of DNA even though it could contain information for more than one kind of polypeptide chain. Matters changed, however, when the sequencing projects revealed still more bizarre phenomena.

Severe cracks in the concept of the gene

In eukaryotic organisms, there are few if any absolute boundaries to transcription, making it impossible to establish simple general relationships between primary transcripts and the ultimate products of those transcripts.

2. Exons of different genes can be members of more than one transcript.

3. Comparably, in the organelles of microbial eukaryotes, many examples of “encrypted” genes are known: genes are often in pieces that can be found as separate segments around the genome.

4. In contradiction to the neoclassical definition of a gene, which posits that the hereditary information resides solely in DNA sequences, there is increasing evidence that the functional status of some genes can be inherited from one generation of individuals to the next, a phenomenon known as transgenerational epigenetic inheritance ( Holliday 1987 ; Gerhart and Kirschner 2007 ; Jablonka and Raz 2009 ).

5. “Genetic restoration” a mechanism of non-Mendelian inheritance of extragenomic information, first found in Arabidopsis thaliana , may also take place ( Lolle et al. 2005 ).

6. Finally, in addition to protein coding genes, there are many RNA-encoding genes that produce diverse RNA molecules that are not translated to proteins.

The observations summarized above, together with many others, have created the interesting situation that the central term of genetics— “the gene”—can no longer be defined in simple terms. The neoclassical molecular definition of the gene does not capture the bewildering variety of hereditary elements, all based in DNA, that collectively specify the organism, and which therefore deserve the appellation of “genes.” Even the classical notion of the gene simply as a fundamental “unit of heredity” is itself problematic. After all, if it is difficult or impossible to generalize about the nature of such “units,” it is probably not very helpful to speak about them. Unsurprisingly, this realization has called forth various attempts to redefine the gene, in terms of both DNA sequence properties, and those of the products specified by those sequences. A number of proposed definitions are listed in Table 1 . A detailed discussion of these ideas will not be given here, but they have been summarized, classified, and characterized (see Waters 2013 ). These definitions, however, all tend to neglect one central, albeit implicit, aspect of the earlier notions of the “gene”: its presumed autonomy of action. We return to this matter below.

Abridged list of different propositions for a definition of the gene in the current era given by different authors

Essential Content or Character of the Proposition .	Classification .	Author(s) .
These three first operational definitions give criteria, formal, experimental and computational, for identifying genes in the DNA sequences of genomes, annotation of genomes, and for specifying the function of genes	Operational
	Operational
	Operational	(2009)
In these three following definitions, classified as molecular, the structural and the functional gene are conceptually distinguished and separated	Molecular
	Molecular
	Molecular
In this definition two gene concepts, “gene-P (preformationist)” and “gene-D (developmental)”, are distinguished	Complex
This definition presents three different concepts of the gene: instrumental, nominal and postgenomic	Complex
This definition aims at to define the gene on the basis of its products and separates it from DNA	A new kind of redefinition

Essential Content or Character of the Proposition .	Classification .	Author(s) .
These three first operational definitions give criteria, formal, experimental and computational, for identifying genes in the DNA sequences of genomes, annotation of genomes, and for specifying the function of genes	Operational
	Operational
	Operational	(2009)
In these three following definitions, classified as molecular, the structural and the functional gene are conceptually distinguished and separated	Molecular
	Molecular
	Molecular
In this definition two gene concepts, “gene-P (preformationist)” and “gene-D (developmental)”, are distinguished	Complex
This definition presents three different concepts of the gene: instrumental, nominal and postgenomic	Complex
This definition aims at to define the gene on the basis of its products and separates it from DNA	A new kind of redefinition

How should geneticists deal with this situation? Should we simply invoke a plurality of different kinds of genes and leave it at that? In effect, we could settle for using the collective term “the genes” as a synonym for the genome, and not fuss over the seeming impossibility of defining the singular form, the “gene.” This, however, would seem to be more of an evasion of the problem than its solution. Alternatively, would it be preferable to accept the inadequacy of the notion of a simple general “unit of heredity,” and foreswear the use of the term “gene” altogether?

The problem with that last suggestion, junking the term “gene,” is not just that the word is used ubiquitously by geneticists and laymen alike, but that it seems indispensable to the discipline’s discourse. This is apparent in the foundations of several subdisciplines of genetics, such as many fields of applied genetics, like medical genetics and plant and animal breeding, that frequently deal in genes identified solely by their nonmolecular mutant phenotypes. It also applies to quantitative genetics and population genetics, which operate using mathematical modeling, and in which the gene is often regarded merely as an abstract unit of calculation (not dissimilarly to the view of Johannsen described below), but one that is vital to conceptualizing the genetic compositions of populations and their changes. In those fields, the molecular intricacies and complications of the genetic material can be largely ignored, at least initially, but the term “gene” itself seems irreplaceable. It is hard to imagine those disciplines abandoning it, whatever the range of molecular complexities that the word both hides and embraces.

In other subdisciplines, such as developmental genetics and molecular genetics, however, there is an urgent need to redefine the gene because the molecular details are often crucial to understanding the phenomena being investigated. The definitions that have been attempted so far ( Table 1 ), however, seem inadequate; for the most part, they focus on either structural or functional aspects, yet it is ultimately meaningless to separate structure and function, even though both can initially be studied in isolation from one another. One attempt to unite the structural and functional aspects of the gene in a single definition has been made by P. E. Griffiths and E. M. Neumann-Held, who introduced the “molecular process” gene concept. In this idea, the word “gene” denotes not some structural “unit of heredity” but the recurring process that leads to the temporally and spatially regulated expression of a particular polypeptide product ( Griffiths and Neumann-Held 1999 ; Neumann-Held 1999 , 2001 ). One difficulty with this redefinition is that it neglects all the nonconventional genes that specify only RNA products. More fundamentally, it has nothing to say about hereditary transmission, which was the original and fundamental impetus for coining the term “gene.”

Perhaps the way forward is to take a step backward in history, and focus on the initial concerns of Johannsen. He not only coined the term “gene,” but was also responsible for the words “genotype” and “phenotype,” and the crucial distinction between them in heredity. Though he could say nothing about how genes (genotype) specified or determined traits (phenotype), he clearly saw this as a crucial question. Indeed, that issue has been at the heart of genetics since the 1930s, in contrast to the questions about how genes are transmitted in heredity, which dominated the first decades of 20th-century genetics. It is apparent, however, that Johannsen thought that the genotype is primary, and that genes are minute computational devices whose precise material nature could be left for solution to a later time. He wrote: “Our formulas, as used here for not directly observable genotypic factors—genes as we used to say—are and remain computational-formulas , placement-devices that should facilitate our overview. It is precisely therefore that the little word “gene” is in place; no imagination of the nature of this “construction” is prejudiced by it, rather the different possibilities remain open from case to case.” ( Johannsen 1926 p. 434, English translation in Falk 2009 p. 70).

The initial expectations were that the connections between genes and phenes would be fairly direct, an expectation bolstered initially by findings about pigmentation genetics, and later by mutations affecting nutritional requirements in microbial cells. In both situations, the connection between the mutant effects and the known biochemistry were often direct and easy to understand. Furthermore, the early success of Mendelian genetics had been based, in large part, on the fact that many of the genetic variants initially studied had constant, unambiguous effects; this was vital to the work of Mendel and to the early 20th-century Mendelians. As the field matured, however, it became apparent that the phenotypic effects of many alleles could be influenced by other genes, influencing both the degree of severity of a mutation’s expression (its “expressivity”), and the proportion of individuals possessing the mutation that expressed it at all (its “penetrance”).

To illustrate the differences in the manifestation of a given gene’s function caused by genetic background effects, take the various degrees of expression of the gene regulating the size and shape of incisors in man. Copies of one dominant gene, identical by descent, caused missing, or peg-shaped, or strongly mesio-distally reduced upper lateral incisors in subsequent generations ( Alvesalo and Portin 1969 ). Though the precise nature of the gene involved is not known, the example shows that the same gene can have different manifestations in different individuals, i.e. , in different genetic backgrounds. There is an enormous number of documented examples of such genetic background effects in all organisms that have been investigated genetically.

The phenomenon of genetic background effects was already well recognized by geneticists in the second decade of the 20th century, as illustrated, for example, in the multi-part series of papers, dealing with coat color inheritance in mammals by S. Wright, published in Journal of Genetics ( Wright 1917a , b ). (Wright would later achieve eminence as one of the key founders of population genetics, but he started his career in what was then known as “physiological genetics.”) The whole matter, however, was raised to a new conceptual level in the 1930s, by C. H. Waddington, a British developmental biologist and geneticist, who called the totality of interactions among genes and between genes and the environment “the epigenotype.”

The epigenotype consists of the total developmental system lying between the genotype and the phenotype through which the adult form of an organism is realized ( Waddington 1939 ). Although a clear concept of “gene regulation” did not exist in the 1940s and 1950s, Waddington, with this concept, was clearly edging toward it. When the Jacob-Monod model of gene regulation came forth in the early 1960s, Waddington promptly saw its relevance for development ( Waddington 1962 ; 1966 ) as, of course, did Jacob and Monod themselves ( Jacob and Monod 1961a , b ; Monod and Jacob 1961 ). The crucial point, with respect to the definition of the gene, is that genes are not autonomous, independent agents—as was implicit in much of the early treatment of genes, and which indeed remains potent in much contemporary thinking, as exemplified in R. Dawkins’ still influential book, “The Selfish Gene” ( Dawkins 1976 ). Rather, they exert their effects within, or as the output of, complex systems of gene interactions. Today, we term such systems “genetic networks” or “genetic regulatory networks” (GRNs). Sewall Wright, along with Waddington, was an early exponent of such network thinking ( Wright 1968 ), but the modern concept of GRNs reached its fruition only in the late 1990s (reviewed in Davidson 2001 ; Wilkins 2002 ; Davidson and Erwin 2006 ; Wilkins 2007 ).

The conceptual consequences of viewing individual genes not as autonomous actors but as interactive elements or outputs of networks are profound. For one thing, it becomes relatively easy to think about the nature of genetic background effects in terms of the structure of GRNs ( Box 2 ). While much of the thinking of the 20th century about genes was based on the premise that the route from gene to phenotype was fairly direct, and often deducible from the nature of the gene product, the network perspective envisages far more complexity and indirectness of effects. In general, the path from particular genes to specific phenes is long, and the role of many gene products seems to be the activation or repression of the activities of other genes. As a result, for most of these interactive effects, the normal (wild-type) function of the gene can only rarely be deduced directly from the mutant phenotype, which often involves complicated secondary effects resulting from the disrupted operation of the GRN within which the gene acts. Hence, the widely held popular belief that particular genes govern or “determine” particular traits, including complex psychological ones ( e.g. , risk-taking, gender identity, autism), as inferred from studies of genetic variants, is a gross oversimplification, hence distortion, of a complex reality.

In effect, genes do not have independent “agency”; for the most part they are simply cogs in the complex machinery of GRNs, and interpreting their mutant phenotypes is often difficult. In contrast, the genes for which there is an obvious connection between the mutant form and an altered phenotype are usually ultimate outputs of GRNs, such as pigmentation genes, hemoglobins, and enzymes of intermediary metabolism. These genes, however, also lack true autonomy, being activated in response to the operation of GRNs. Therefore, to fully understand how a gene functions, one must comprehend the larger systems in which they operate. Genetics, in this sense, is becoming systems biology, a point that has also been made by others (see, for example, Keller 2005 ). In effect, since genes can only be defined with respect to their products, and those products are governed by GRNs, the particular cellular and regulatory (GRN) contexts involved may be considered additional “dimensions” vital to specifying a gene’s function and identity. The examples of “gene sharing,” in which the function of the gene is wholly a function of its cellular context, illustrate this in a particularly vivid way. The “gene”—however it comes to be defined—can therefore be seen not as a three-dimensional entity but as a multi-dimensional one.

Where do all these considerations leave us? It took approximately half a century to go from Johannsen’s wholly abstract formulation of the term “gene” as a “unit of heredity,” to reach the early 1960s concept of the gene as a continuous segment of DNA sequence specifying a polypeptide chain. A further half century’s worth of experimental investigation has brought us to the realization that the 1960s definition is no longer adequate as a general one. Yet the term “gene” persists as a vaguely understood generic description. It is, to say the least, an anomalous situation that the central term of genetics should now be shrouded in confusion and ambiguity. That is not only intellectually unsatisfactory for the discipline, but has detrimental effects on the popular understanding of genetics. Such misunderstanding is seen most starkly in the situation noted earlier, the commonly held view that there are individual genes responsible “for” certain complex conditions, e.g. , schizophrenia, alcoholism, etc. A clearer definition of the term would thus help both the field of genetics, and, ultimately, public understanding.

Here, therefore, we will propose a definition that we believe comes closer to doing justice to the idea of the “gene,” in light of current knowledge. It makes no reference to “the unit of heredity”—the long-standing sense of the term—because we feel that it is now clear that no such generic universal unit exists. By referring to DNA sequences, however, our definition embodies the hereditary dimension of genes (in a way that pure “process”-centered definitions focused on gene expression do not). Furthermore, in its emphasis on the ultimate molecular products and reference to GRNs as both evokers and mediators of the actions of those products, it recognizes the long causal chains that often operate between genes and their effects. Our provisional definition is this:

A gene is a DNA sequence (whose component segments do not necessarily need to be physically contiguous) that specifies one or more sequence-related RNAs/proteins that are both evoked by GRNs and participate as elements in GRNs , often with indirect effects , or as outputs of GRNs , the latter yielding more direct phenotypic effects .

This is an explicitly “molecular” definition, but we think that is what is needed now. In contrast, “genes” that are identified purely by their phenotypic effects, as for example in genome-wide association study (GWAS) experiments, would, in our view, not deserve such a characterization until found to specify one or more RNAs/proteins. The genetic effects picked up in such work often identify purely regulatory elements, and these should not qualify as genes, only as part of genes. Our definition, like the classic 1960s’ formulation, makes identifying the product(s) crucial to delimiting, hence identifying, the genes themselves. It, however, also emphasizes the molecular and cellular context in which those products form and function. Those larger contexts, in effect, become necessary to define the function of the specifying gene(s).

The new definition, however, is slightly cumbersome. We therefore offer it only as a tentative solution, hence as a challenge to the field to find a better formulation but one that does justice to the complex realities of the genetic material uncovered in the past half-century.

Of fundamental importance in the operational definition of the gene is the cis-trans test ( Lewis 1951 ; Benzer 1957 ). To test whether mutations a and b belong to the same gene or cistron ( Benzer 1957 ), or different cistrons, the cis -heterozygote a b/+ + and the trans -heterozygote a +/+ b are compared. If the cis -heterozygotes, and the trans -heterozygotes are phenotypically similar (usually wild type), they are said to “complement” one another, and the mutations are inferred to fall into different cistrons. If, however, the cis -heterozygotes and the trans -heterozygotes are phenotypically different, the trans -heterozygote being (usually) mutant, and the cis -heterozygote (usually) of wild type, the mutations do not complement, and are inferred to belong to the same cistron. The attached figure clarifies the idea.

The principle of the cis-trans test. If mutations a and b belong to the same cistron, the phenotypes of the cis - and trans -heterozygotes are different. If, however, the cis - and trans -heterozygotes are phenotypically similar, the mutations a and b belong to different cistrons. The notation “works” on the Figure means that the cistron is able to produce a functional polypeptide. Mutations a and b are recessive mutations that both affect the same phenotypic trait, such as the eye color of D. melanogaster , for example.

Genetic background effects typically exhibit either of two forms, when a pre-existing mutation, with an associated phenotypic manifestation, is crossed into a different strain: the reduction (“suppression”) of the mutant phenotype or its increase (“enhancement”). The effects involve either changes in the degree (“expressivity”) of the mutant effect, or the number of individuals) affected (its “penetrance”), or both. When analyzed genetically, these effects could often be traced to specific “suppressor” or “enhancer” loci, which could be either tightly linked or distant in the genome from the original mutant locus. Typically regarded as an unnecessary complication in analysis of the original mutation, they were usually not pursued further. Yet, in terms of current understanding of GRNs, they are not, in principle, mysterious. Each gene that is part of a GRN can be thought of as either transmitting a signal for the activation or repression of one or more other “downstream” genes in that network, but, given the hierarchical nature of GRNs, it follows that a mutational alteration in a specific gene in the network can be either strengthened or reduced by other mutational changes in the network, either upstream or downstream of the original mutation. The particular effect achieved will depend on the characteristics of each of the two mutations involved—whether they are loss-of- or gain-of-function mutations—and the precise nature of their connectivity. Such effects are most readily illustrated with linear sequences of gene actions, genetic pathways ( Wilkins 2007 ), but can be understood in networks, when the network structure and the placement of the two genes within them is known. Some genetic background effects, in principal, however, might involve partially redundant networks, in which the effects of the two pathways are additive. In those cases, a mutant effect in one pathway may be either compensated, hence suppressed, or exacerbated, by a second mutation in the other pathway, the precise effects again depending upon the specific characteristics of the mutations and the degree of redundancy between the two GRNs.

We thank Mark Johnston and Richard Burian for many helpful suggestions, both editorial and substantive, on previous drafts. A.W. would also like to acknowledge earlier conversations with Jean Deutsch on the subject of this article; we disagreed on much but the process was stimulating and helpful. P.P. wants to thank his friends Marja Vieno, M.Sc. for linguistic aid at the very first stages of this project, and Harri Savilahti, Ph.D. for a fruitful discussion, and Docent Mikko Frilander, Ph.D. for consultation. The authors declare no conflict of interest.

Communicating editor: M. Johnston

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greylag. Flock of Greylag geese during their winter migration at Bosque del Apache National Refugee, New Mexico. greylag goose (Anser anser)

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Auburn University - College of Sciences and Mathematics - Choosing Mates: Good Genes Versus Genes that are a Good Fit

good genes hypothesis , in biology , an explanation which suggests that the traits females choose when selecting a mate are honest indicators of the male’s ability to pass on genes that will increase the survival or reproductive success of her offspring. Although no completely unambiguous examples are known, evidence supporting the good genes hypothesis is accumulating, primarily through the discovery of male traits that are simultaneously preferred by females and correlated with increased offspring survival. For example, female North American house finches ( Carpodacus mexicanus ) prefer to mate with bright, colourful males. Such male finches also have high overwinter survivorship. This preference suggests that mating with such males will increase offspring survival. British evolutionary biologist W.D. Hamilton and American behavioral ecologist Marlene Zuk first proposed this hypothesis in the early 1980s.

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Lyon hypothesis

Medical Definition of Lyon hypothesis

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“Lyon hypothesis.” Merriam-Webster.com Medical Dictionary , Merriam-Webster, https://www.merriam-webster.com/medical/Lyon%20hypothesis. Accessed 29 Jun. 2024.

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What Is A Hypothesis
Hypothesis Examples
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Hypothesis
definition of genetic code # wobble hypothesis #genetics #biology # class +2
Hypothesis Examples

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Concept of Hypothesis
BARR BODY AND LYON'S HYPOTHESIS (GENETICS-1/13)
Hypothesis: meaning Definition #hypothesis #statistics #statisticsforeconomics #statisticalanalysis
Define hypothesis
What Is A Hypothesis?
Biological Method

COMMENTS

Genetics and Statistical Analysis
The key is statistical examination, which allows you to determine whether your data are consistent with your hypothesis. For instance, when performing a genetic cross, the chi-square test allows ...
Hypothesis
A hypothesis is a supposition or tentative explanation for (a group of) phenomena, (a set of) facts, or a scientific inquiry that may be tested, verified or answered by further investigation or methodological experiment. It is like a scientific guess. It's an idea or prediction that scientists make before they do experiments.
Probabilities in genetics (article)
It reflects the number of times an event is expected to occur relative to the number of times it could possibly occur. For instance, if you had a pea plant heterozygous for a seed shape gene ( Rr) and let it self-fertilize, you could use the rules of probability and your knowledge of genetics to predict that 1. ‍.
1.13: Introduction to Mendelian Genetics
Introduction. In plant and animal genetics research, the decisions a scientist will make are based on a high level of confidence in the predictable inheritance of the genes that control the trait being studied. This confidence comes from a past discovery by a biologist named Gregor Mendel, who explained the inheritance of trait variation using ...
Using genetics to understand biology
It requires detailed hypothesis testing, and experimentation that combines genetics, biochemistry and cell biology. As data accumulate, it should be possible to develop systematic, theoretical and ...
The Evolving Definition of the Term "Gene"
Abstract. This paper presents a history of the changing meanings of the term "gene," over more than a century, and a discussion of why this word, so crucial to genetics, needs redefinition today. In this account, the first two phases of 20th century genetics are designated the "classical" and the "neoclassical" periods, and the ...
Introduction to heredity review (article)
Gregor Mendel's principles of heredity, observed through patterns of inheritance in pea plants, form the basis of modern genetics. Mendel proposed that traits were specified by "heritable elements" called genes. Genes come in different versions, or alleles, with dominant alleles being expressed over recessive alleles.
Gregor Mendel and the Principles of Inheritance
By experimenting with pea plant breeding, Mendel developed three principles of inheritance that described the transmission of genetic traits, before anyone knew genes existed. Mendel's insight ...
Scientific hypothesis
hypothesis. science. scientific hypothesis, an idea that proposes a tentative explanation about a phenomenon or a narrow set of phenomena observed in the natural world. The two primary features of a scientific hypothesis are falsifiability and testability, which are reflected in an "If…then" statement summarizing the idea and in the ...
Genetics
Genetics forms one of the central pillars of biology and overlaps with many other areas, such as agriculture, medicine, and biotechnology. Since the dawn of civilization, humankind has recognized the influence of heredity and applied its principles to the improvement of cultivated crops and domestic animals. A Babylonian tablet more than 6,000 ...
Variations on Mendel's laws (overview) (article)
Other variations on Mendel's rules involve interactions between pairs (or, potentially, larger numbers) of genes. Many characteristics are controlled by more than one gene, and when two genes affect the same process, they can interact with each other in a variety of different ways. For example: Complementary genes.
22.2: Population Genetics
Hypothesis: Repeated natural disasters will yield different population genetic structures; therefore, each time this experiment is run, the results will vary. Test the hypothesis: Count out the original population using different colored beads. For example, red, blue, and yellow beads might represent red, blue, and yellow individuals.
Multiple Factor Hypothesis (With Example)
ADVERTISEMENTS: This is the essence of the multiple factor hypothesis. As quantitative inheritance it is controlled by many genes. Therefore, it is also known as polygenic inheritance. A few common examples of polygenic inheritance are described as below: Seed colour in Wheat:
Genome, Genes, DNA, and Chromosomes: Basics of Genetics
In the simplest terms, a genome is the complete set of genetic instructions that determine the traits (characteristics and conditions) of an organism. It is made up of DNA, genes, and chromosomes. DNA is a molecule in cells that carries the genetic information. It is made up of building blocks. The genetic coding of our traits is based on how ...
Hypothesis
hypothesis, something supposed or taken for granted, with the object of following out its consequences (Greek hypothesis, "a putting under," the Latin equivalent being suppositio ). Discussion with Kara Rogers of how the scientific model is used to test a hypothesis or represent a theory. Kara Rogers, senior biomedical sciences editor of ...
Genetics
Genetics is the study of genes and the variation of characteristics that are influenced by genes—including physical and psychological characteristics. ... The new Biophilia Reactivity Hypothesis ...
Wobble Hypothesis (With Diagram)
In this article we will discuss about the concept of wobble hypothesis. Crick (1966) proposed the 'wobble hypothesis' to explain the degeneracy of the genetic code. Except for tryptophan and methionine, more than one codons direct the synthesis of one amino acid. There are 61 codons that synthesise amino acids, therefore, there must be 61 ...
hypothesis noun
a speculative hypothesis concerning the nature of matter; an interesting hypothesis about the development of language; Advances in genetics seem to confirm these hypotheses. His hypothesis about what dreams mean provoked a lot of debate. Research supports the hypothesis that language skills are centred in the left side of the brain.
Evolving Definition of the Term "Gene"
This paper presents a history of the changing meanings of the term "gene," over more than a century, and a discussion of why this word, so crucial to genetics, needs redefinition today. In this account, the first two phases of 20th century genetics are designated the "classical" and the "neoclassical" periods, and the current ...
Good genes hypothesis
good genes hypothesis, in biology, an explanation which suggests that the traits females choose when selecting a mate are honest indicators of the male's ability to pass on genes that will increase the survival or reproductive success of her offspring. Although no completely unambiguous examples are known, evidence supporting the good genes hypothesis is accumulating, primarily through the ...
Hypothesis Definition & Meaning
hypothesis: [noun] an assumption or concession made for the sake of argument. an interpretation of a practical situation or condition taken as the ground for action.
Particulate inheritance
Gregor Mendel, the Father of Genetics William Bateson Ronald Fisher. Particulate inheritance is a pattern of inheritance discovered by Mendelian genetics theorists, such as William Bateson, Ronald Fisher or Gregor Mendel himself, showing that phenotypic traits can be passed from generation to generation through "discrete particles" known as genes, which can keep their ability to be expressed ...
Lyon hypothesis Definition & Meaning
Ly· on hypothesis ˈlī-ən-. : a hypothesis explaining why the phenotypic effect of the X chromosome is the same in the mammalian female which has two X chromosomes as it is in the male which has only one X chromosome: one of each two somatic X chromosomes in mammalian females is selected at random and inactivated early in embryonic development.