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).

Step-by-step solution

Understanding Heritability

Heritability is a measure of the proportion of phenotypic variation in a population that can be attributed to genetic variation. It helps in understanding how much of an organism's traits are influenced by its genes. There are two types of heritability estimates: broad-sense heritability (H^2) and narrow-sense heritability (h^2).

Broad-Sense Heritability

Broad-sense heritability (H^2) refers to the proportion of phenotypic variance that can be attributed to the total genetic variance, including additive genetic effects (A), dominance genetic effects (D), and interaction genetic effects (I). H^2 can be defined mathematically as: H^2 = (V_A + V_D + V_I) / V_P where V_A is the additive genetic variance, V_D is the dominance genetic variance, V_I is the interaction genetic variance, and V_P is the phenotypic variance.

Narrow-Sense Heritability

Narrow-sense heritability (h^2) refers to the proportion of phenotypic variance that can be attributed only to the additive genetic variance. It is the proportion of phenotypic differences that can be transmitted from parents to offspring. h^2 can be defined mathematically as: h^2 = V_A / V_P where V_A is the additive genetic variance, and V_P is the phenotypic variance.

Understanding Twin Studies

Twin studies are widely used in behavioral genetics to investigate the heritability of human traits. In these studies, monozygotic (identical) twins and dizygotic (fraternal) twins are compared. Monozygotic twins share 100% of their genes, while dizygotic twins share, on average, 50% of their segregating genes. By comparing the similarities and differences in various traits between the twin pairs, researchers can estimate the heritability of those traits.

Heritability Estimates from Twin Studies

Twin studies primarily estimate narrow-sense heritability (h^2) based on the assumption that environmental effects are shared equally between twins. Through twin studies, scientists can separate the effects of shared environmental factors (C) and unique environmental factors (E), which helps them determine the additive genetic influence on a trait. By comparing the similarities between identical and fraternal twins, they can estimate the proportion of the phenotypic variance that is due to the additive genetic variance, which represents narrow-sense heritability (h^2). To conclude, human twin studies provide estimates of narrow-sense heritability (h^2).

Key concepts

Broad-Sense Heritability (H^2)

Broad-sense heritability, symbolized as \(H^2\), is a key concept in the field of genetics. It describes the proportion of phenotypic variance that can be attributed to all types of genetic variance in a population.

When we talk about phenotypic variance (\(V_P\)), we are referring to the observable variation in a trait across individuals. This variation arises from both genetic factors and environmental influences. The genetic component of this variance consists of several elements, including additive effects (\(V_A\)), dominance effects (\(V_D\)), and gene-gene interaction effects, also known as epistatic interactions (\(V_I\)).

The formula for calculating broad-sense heritability is \(H^2 = \frac{V_A + V_D + V_I}{V_P}\). A high \(H^2\) value suggests that genetic factors play a significant role in the observed variation of a trait, while a lower value indicates a greater influence of environmental factors. Understanding \(H^2\) is crucial for breeders and researchers in selecting traits for improvement and in studying the nature of genetic inheritance.

Narrow-Sense Heritability (h^2)

Narrow-sense heritability, denoted as \(h^2\), narrows down the focus of genetic influence to additive genetic variance only. This type of heritability estimate is particularly important in understanding how traits are passed from parents to offspring.

It is the proportion of the total phenotypic variance (\(V_P\)) due to the additive effects (\(V_A\)) of alleles. Additive genetic variance refers to the cumulative effect of individual alleles contributing to a trait, where the overall effect is the sum of the effects of all alleles. The mathematical expression for narrow-sense heritability is \(h^2 = \frac{V_A}{V_P}\).

This measurement is invaluable in breeding programs and evolutionary biology since it reflects the genetic potential for a population's trait values to respond to natural or artificial selection. When \(h^2\) is high, selective breeding is more likely to be successful, as offspring tend to resemble their parents for the trait in question.

Genetic Variance

Genetic variance is the portion of the total phenotypic variance that is due to differences in genes among individuals in a population. It is often symbolized as \(V_G\) and comprises several types, including additive variance (\(V_A\)), dominance variance (\(V_D\)), and gene-gene interaction variance (\(V_I\)).

• \(V_A\): Additive variance is the effect of individual alleles summed across all loci that affect the trait.
• \(V_D\): Dominance variance arises from interactions between alleles at the same locus.
• \(V_I\): Interaction variance, also known as epistasis, occurs from interactions between alleles at different loci.

This subdivision of genetic variance is essential to understanding the complexity of trait inheritance and predicting the response of traits to selection in animal and plant breeding or in natural evolutionary processes.

Phenotypic Variance

Phenotypic variance, represented as \(V_P\), captures the overall variation observed in a trait within a population. It encompasses all the variations that we can measure or observe, from height to intelligence, and is a result of both genetic factors and environmental influences.

Essentially, \(V_P\) is the sum of genetic variance (\(V_G\)) and environmental variance (\(V_E\)). Environmental variance includes any non-genetic influences that can affect a trait, such as diet, climate, and personal experiences.

Quantifying phenotypic variance is a fundamental step in determining heritability. By understanding the magnitude of \(V_P\) and its components, researchers can gauge how much of the variation can potentially be passed to the next generation and how much is due to environmental factors. This information is critical for making informed decisions in genetics, evolution, and conservation biology.