Question

Using a simple group, mother and offspring, as an example, explain how direct genetic effects and indirect genetic effects can contribute to a phenotypic trait such as offspring growth rate and can be used to estimate the heritability of a group phenotypic trait. You need to mention additive genetic variance in your answer.

Answer 1

• Heritability is a commonly used and important term to describe properties of the inheritance. Informally, heritability (h2) is the proportion of the variation in the trait due to genetic differences between individuals. h2 is defined as the proportion of the phenotypic variance caused by additive genes, thereby excluding genetic variance controlled by dominance and epistasis (non-additive genetic variance).. Correlations among relatives and response to directional selection are proportional to the heritability. Although a property of a specific trait in a specific population, it is found that heritabilities of similar traits take similar values in different species and populations.

• Indirect genetic effects (IGE) occur when the genotype of an individual affects the phenotypic trait value of another conspecific individual. The classical example of an IGE is the maternal genetic effect of a mother on the trait values of her offspring in a mammal .

• In the context of the classical quantitative genetic model where the trait value of an individual is decomposed into the heritable effect of its genotype and a residual labelled as environment, P=G+E, the E-term is partly heritable when IGEs occur. Consequently, compared with predictions based on the classical model, IGEs may substantially alter response to selection, both in magnitude and direction

Evolutionary responses to selection require that traits have a heritable basis, yet maternal effects (the effect of a mother’s phenotype on her offspring’s phenotype) can have profound effects on evolutionary processes.

In addition to the direct genetic contribution from the mother to her offspring (i.e., through additive genetic effects), mothers also can indirectly influence variance in offspring phenotypes through maternal effects. For example, mothers may manipulate the offspring’s developmental environment and/or differentially invest into offspring depending on their phenotype . Theoretical work has shown that maternal effects can have important and even counterintuitive effects on the response to selection and possibly facilitate the maintenance of additive genetic variation. Maternal effects are known to influence a diversity of important offspring traits including body size and mass, performance traits, and growth rates