AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

DEFINITION

Narrow sense

• Breeding value is of prime importance in selection programme since breeding values are passed on from parent to offspring and not the genotype. Therefore we are interested in the proportion of phenotypic variance attributable to breeding values. This is represented by the fraction of V_A/V_P, which is called the heritability in the narrow sense (h²).
• Heritability is defined as the ratio of additive genetic variance to phenotypic variance.

Heritability in narrow sense is used to refer per cent or proportion of the phenotypic variation between individuals for a particular trait that is due to differences in the additive genetic effects of the trait.

h² = V_A/V_P

• The ratio V_A/V_P expresses the extent to which phenotypes are determined by the genes transmitted from parents. It represents the percentage of genetic progress made in the next generation when superior individuals are selected as parents.

Broad sense

• The ratio V_G/V_P is called the heritability in the broad sense or the degree of genetic determination.
• It expresses the extent to which individual’s phenotypes are determined by the genotypes. Heritability in broad sense includes variation due to additive gene action (V_A), dominance (V_D) and epistasis (V_I).
• In the selection programme it is the heritability in the narrow sense or simply heritability, which is very important. Unless otherwise specified, heritability means heritability in the narrow sense (h²).

Symbol

• The customary symbol h² stands for the heritability itself and not for its square.
• The symbol derives from Wright's (1921) terminology where “h” stands for the corresponding ratio of standard deviations.

Mathematically heritability

• Mathematically heritability is the regression of breeding value on phenotypic value. h² = b_AP. Therefore h² can be taken as the change in breeding value expected per unit change in phenotypic value.