Question

Define the term broad-sense heritability \(\left(H^{2}\right) .\) What is implied by a relatively high value of \(H^{2}\) ? Express aspects of broad-sense heritability in equation form.

Short answer

Answer: A high value of broad-sense heritability indicates that genetic factors have a significant influence on the observed variation in a particular trait within the population, and that the trait is more likely to respond to selection pressures. Broad-sense heritability can be expressed in equation form as: $$ H^2 = \frac{V_G}{V_P} $$ Where \(H^2\) is broad-sense heritability, \(V_G\) is genetic variance, and \(V_P\) is phenotypic variance.

Step-by-step solution

Define broad-sense heritability

Broad-sense heritability (\(H^2\)) is a measure of the proportion of phenotypic variance that can be attributed to genetic variance in a population. It helps estimate how much of the observed variation in a trait is due to genetic factors rather than environmental factors. It is expressed as a value between 0 and 1, with 0 meaning no genetic contribution and 1 meaning full genetic contribution.

Explain the implications of a high value of \(H^2\)

A relatively high value of \(H^2\) implies that the genetic factors have a significant influence on the observed variation in a particular trait within the population. It also suggests that the trait will be more likely to respond to artificial or natural selection because traits with high heritability are generally transmitted more faithfully across generations, allowing them to change faster in response to selection pressures.

Express aspects of broad-sense heritability in equation form

In order to express the aspects of broad-sense heritability in equation form, we can write the equation as: $$ H^2 = \frac{V_G}{V_P} $$ Where: - \(H^2\): Broad-sense heritability - \(V_G\): Genetic variance (variation attributed to genetic factors) - \(V_P\): Phenotypic variance (total observed variation in the trait) Broad-sense heritability can also be partitioned into its components, which include additive genetic variance (\(V_A\)), dominance genetic variance (\(V_D\)), and interaction genetic variance (\(V_I\)). In this case, the equation would be: $$ H^2 = \frac{V_A + V_D + V_I}{V_P} $$

Key concepts

Genetic Variance

Understanding genetic variance is crucial when studying the hereditary aspects of traits in a population. Essentially, genetic variance refers to the diversity of gene alleles within a population, which can result in observable differences in physical features, abilities, and behaviors. It is the variation that occurs solely because of the different combinations of genes present in an organism.

Imagine a classroom where students have different heights. The variations in height that are passed down from parents to children due to genetic factors represent the genetic variance in that trait. If one parent is tall and the other short, the range of heights observed in their offspring is a direct example of genetic variance.

This variability is the raw material on which natural selection acts. It is also essential in breeding programs, where specific traits are selected to be passed down to future generations.

Phenotypic Variance

Phenotypic variance, denoted by \(V_P\), encompasses the total variation in a trait observed in a population. This variance includes the physical expressions or phenotypes of all the individual organisms. Unlike genetic variance, phenotypic variance reflects both inherited genetic factors and environmental influences that affect development and trait expression.

For example, the height of a plant can vary due to genetic differences (seed type) and environmental factors such as the quality of the soil, access to sunlight, and availability of water. Even genetically identical plants will exhibit some differences in height when grown under varying environmental conditions.

Understanding this distinction is critical for determining how much control we have over changing those traits through interventions. In other words, it helps us understand what traits can be influenced through genetic selection versus environmental management.

Heritability Equation

The heritability equation is a formula that serves as a quantitative measure of the proportion of total phenotypic variance in a trait that can be attributed to genetic factors. In its simplest form, the broad-sense heritability equation is represented as \(H^2 = \frac{V_G}{V_P}\).

In this equation, \(H^2\) signifies broad-sense heritability, \(V_G\) represents the genetic variance, and \(V_P\) is the phenotypic variance. This equation implies that if we know the total variance of a trait and can quantify how much of that variance is due to genetics, we can compute the broad-sense heritability. The closer \(H^2\) is to 1, the stronger the genetic basis for the trait. A lower \(H^2\) value indicates a larger impact of the environment on the trait's variance.

Additive Genetic Variance

Additive genetic variance, denoted as \(V_A\), is an essential part of genetic variance that directly affects the traits passed from parents to offspring. It is the variance that can be attributed to the sum of the average effects of individual alleles, which are the different versions of a gene.

This aspect of variance is particularly important for breeders because it determines the response to selection over generations. If a trait has high additive genetic variance, it means that selecting and breeding individuals with the desired traits will likely result in offspring that also exhibit those traits more reliably. In contrast, if additive genetic variance is low, significant improvement of a trait through selective breeding is less likely.

Notably, the broad-sense heritability equation can be expanded to also include dominance variance (\(V_D\)), which occurs when an allele's effect is different depending on the presence of other alleles, and interaction variance (\(V_I\)), which is the variance from interactions among alleles at different genes.