Final answer:
Narrow-sense heritability will decrease if dominance variance increases while other variance components are constant, as it implies a lower proportion of phenotypic variance attributable to additive genetic factors.
Step-by-step explanation:
If a characteristic has a narrow-sense heritability of 0.6, and the dominance variance (VD) increases while all other variance components remain the same, the narrow-sense heritability will decrease. This is because narrow-sense heritability (also known as heritability in the narrow sense) is calculated as the proportion of the total phenotypic variance that is due to additive genetic variance (VA), which includes the effects of individual alleles that add up or average out in the population.
Heritability in the narrow sense is given by the formula h^2 = VA / (VA + VD + VE), where VA is the additive genetic variance, VD is the dominance variance, and VE is the environmental variance. When VD increases, the denominator of the fraction increases while the numerator, VA, remains the same, leading to a smaller fraction, hence a lower narrow-sense heritability. It's essential to remember that heritability measures the proportion of observed variation in a phenotype that is due to genetic factors when holding the environment constant. An increase in dominance variance indicates a greater contribution of dominance interactions to the phenotype, thereby diminishing the proportion of phenotype variance attributable solely to additive genetic factors.