The nature of heritability

Summary

This video explores the concept of heritability in genetics, explaining how genetic factors contribute to traits like height in humans and weight in chickens. It discusses selection experiments, using chickens as an example, to illustrate how heritability is measured by the response to selection. The video also examines twin studies to estimate the genetic contribution to human traits such as sexual orientation. Key concepts include genetic variance, phenotypic variance, and the distinction between narrow-sense and broad-sense heritability. It highlights how environmental factors can influence heritability estimates across generations and populations.

Takeaways

• 😀 Heritability refers to the genetic contribution to a trait, such as human height, and is measured through the response to selection in a breeding experiment.
• 😀 Selection experiments, like breeding the heaviest chickens, can help estimate the heritability of a trait by comparing the mean of the breeding population with the mean of the offspring population.
• 😀 Perfect response to selection occurs when the offspring mean matches the mean of the breeding population, indicating high heritability (heritability = 1).
• 😀 If there is no response to selection, heritability is considered to be 0, meaning the trait does not show genetic variation that responds to selection.
• 😀 Narrow-sense heritability quantifies the proportion of genetic variation in a trait that is additive and thus capable of responding to selection.
• 😀 Traits closely related to fitness, such as egg number in chickens, tend to have lower heritability, as they have already undergone selection in previous generations.
• 😀 Traits like egg weight show higher heritability because they may not have as strong a direct impact on overall fitness and therefore have more genetic variation available for selection.
• 😀 Ethical concerns prevent similar breeding experiments in humans, but twin studies provide an alternative way to estimate heritability by comparing monozygotic (identical) and dizygotic (fraternal) twins.
• 😀 Twin studies have been used to investigate the heritability of complex traits such as sexual orientation, with results showing that monozygotic twins are more likely to share similar traits compared to dizygotic twins.
• 😀 Variance measures the spread of a trait's distribution, and heritability can be calculated as the ratio of genetic variance to phenotypic variance, where phenotypic variance includes both genetic and environmental contributions.
• 😀 Genetic variance consists of additive genetic variance (which responds to selection), dominance variance (which does not respond as strongly), and interaction variance (which arises from interactions between alleles at different loci).

Q & A

What is heritability, and how is it measured?

Heritability is the proportion of variation in a trait that is attributable to genetic differences among individuals. It is measured by comparing the mean traits of a breeding population and their offspring. The response to selection is divided by the selection intensity to calculate heritability. If offspring characteristics match the parent population perfectly, heritability is 1; if there is no change, it is 0.

What does the experiment with chickens tell us about heritability?

The chicken experiment illustrates how selective breeding can be used to measure heritability. By selecting only the heaviest chickens to breed, researchers can assess how much the trait (in this case, weight) responds to selection. If the offspring's average weight shifts toward the breeding population, heritability is high. If there's no shift, heritability is low.

What is the difference between narrow-sense and broad-sense heritability?

Narrow-sense heritability refers to the proportion of genetic variation that responds to selection, focusing on additive genetic variance. Broad-sense heritability includes all genetic contributions, including additive, dominance, and interaction effects, but only the additive component is relevant for selection.

Why is there a difference in heritability between egg number and egg weight in chickens?

Egg number has a lower heritability because it is more closely linked to overall fitness. Traits that impact reproductive success, like egg number, have already been optimized by natural selection, leaving less genetic variation to select from. Egg weight, on the other hand, is less directly related to fitness, allowing for more genetic variation and, therefore, a higher heritability.

Why are experiments involving human genetics, like those with chickens, ethically problematic?

Human genetic experiments that involve selective breeding to influence traits raise significant ethical concerns, including the implications of controlling reproduction. These experiments can infringe on individual rights, freedom, and personal autonomy. For this reason, such practices are considered unethical in humans.

How can we study heritability in humans if breeding experiments are not ethical?

In humans, heritability is often studied through twin studies, comparing monozygotic (identical) twins with dizygotic (fraternal) twins. These studies can estimate the genetic contribution to a trait by observing similarities and differences between twins who share varying degrees of genetic relatedness.

What do twin studies suggest about the genetic contribution to sexual orientation?

Twin studies have shown that there is a genetic component to sexual orientation, with higher concordance rates for sexual orientation among monozygotic twins compared to dizygotic twins. However, environmental factors also play a role, as heritability estimates for sexual orientation vary depending on societal acceptance and other factors.

What is the role of environmental variation in heritability estimates?

Environmental variation can influence heritability estimates by adding to the phenotypic variation. If environmental factors such as nutrition or societal attitudes change over time, they can affect the trait's expression, which in turn impacts the heritability estimate. As environmental variation increases or decreases, the heritability of a trait can change.

What is variance, and how does it relate to heritability?

Variance measures the spread or distribution of a trait around the mean. It quantifies how much individual values deviate from the population average. In the context of heritability, genetic variance and environmental variance contribute to the total phenotypic variance. Heritability is the proportion of total variance that can be attributed to genetic differences.

How does genetic variance contribute to phenotypic variance?

Genetic variance contributes to phenotypic variance by determining the potential for individuals to express traits in different ways. While environmental factors also affect phenotypic expression, the genetic variance represents the potential inherited differences that can be passed down across generations. These genetic factors are key to the heritability of a trait.