Question

What kind of heritability estimates (broad sense or narrow sense) are obtained from human twin studies?

Short answer

Answer: Human twin studies provide estimates of narrow sense heritability (h^2) because they focus on the portion of phenotypic variation that is attributable to additive genetic effects. This is achieved by comparing the phenotypic similarities between monozygotic and dizygotic twins, while non-additive effects, such as dominance and epistasis, are not precisely captured.

Step-by-step solution

Understanding Broad Sense and Narrow Sense Heritability

Broad sense heritability (H^2) is the proportion of phenotypic variation in a population that is attributable to genetic variation. It includes all genetic effects (additive, dominance, and interaction). Narrow sense heritability (h^2) refers to the proportion of phenotypic variation that is due to additive genetic effects only. Additive genetic effects are those that can be passed from parents to offspring and can be used to predict the response to selection.

Understanding Twin Studies

Twin studies are used to estimate heritability by comparing the phenotypic similarities between monozygotic (identical) twins and dizygotic (fraternal) twins. Monozygotic twins share 100% of their genes, while dizygotic twins share, on average, 50% of their genes. By comparing the phenotypic similarities of these two types of twins, researchers can estimate the heritability of a particular trait.

Heritability Estimates from Twin Studies

Twin studies provide estimates of narrow sense heritability (h^2). This is because the additive genetic effects are directly measured by comparing the phenotypic similarities between monozygotic and dizygotic twins, while non-additive effects (such as dominance and epistasis) are not precisely captured. By comparing the phenotypic correlations between monozygotic and dizygotic twins, researchers can estimate the proportion of phenotypic variation due to additive genetic effects, which is the narrow sense heritability. To sum up, human twin studies are suited for obtaining narrow sense heritability estimates since they focus on the portion of phenotypic variation that is attributable to additive genetic effects.

Key concepts

Twin Studies

Twin studies are a crucial tool in genetics for understanding the heritability of traits. They involve the comparison of identical (monozygotic) twins and fraternal (dizygotic) twins to determine how much of a trait's variation is due to genetics.

Monozygotic twins share all of their genes because they are formed from the same fertilized egg. In contrast, dizygotic twins, like typical siblings, share about 50% of their genes since they come from two separate fertilized eggs.

By examining how similar or different these twins are in terms of specific traits, scientists can tease apart the genetic versus the environmental influences on those traits.

Broad Sense Heritability

Broad sense heritability, represented as \(H^2\), accounts for all genetic variance in a population. This includes three types of genetic factors: additive, dominance, and gene-gene interactions (epistatic).

In simpler terms, it measures the extent to which genetic factors contribute to phenotypic variance.

However, it does have its limitations. Since it encompasses all genetic effects, it doesn't differentiate which specific genetic contributions are more influential on a trait. It's more of a big-picture estimate, useful for considering the total potential genetic influence on traits.

Narrow Sense Heritability

Narrow sense heritability, designated as \(h^2\), is a more specific measure than broad sense heritability. It focuses solely on additive genetic effects, which are the genetic factors that directly pass from parents to offspring.

These additive effects are important because they are stable and predictable, allowing for better estimations of change due to selection in breeding programs.

This type of heritability is particularly valuable in twin studies, where it helps determine how much of the trait variation between individuals is due to the simple additive effects of shared genes.

Additive Genetic Effects

Additive genetic effects form the foundation of narrow sense heritability. They are essentially the effects of individual alleles that sum up to influence a trait. Unlike dominance or epistatic effects, additive effects are consistent across generations.

This consistency means that these effects can reliably predict the outcomes of selective breeding.

In the context of twin studies, focusing on additive genetic effects allows researchers to clearly identify how much of a trait's variability is due to straightforward, inherited genetic factors, as opposed to more complex genetic interactions or environmental influences.