The Calculation of Heritability

Having considered the components that contribute to phe-notypic variance and having developed a general concept of heritability, we can ask, How does one go about estimating these different components and calculating heritability? There are several ways to measure the heritability of a characteristic. They include eliminating one or more variance components, comparing the resemblance of parents and offspring, comparing the phenotypic variances of individuals with different degrees of relatedness, and measuring the response to selection. The mathematical theory that underlies these calculations of heritability is complex and beyond the scope of this book. Nevertheless, we can develop a general understanding of how heritability is measured.

Heritability by elimination of variance components

One way of calculating the broad-sense heritability is to eliminate one of the variance components. We have seen that VP = VG + VE + VGE. If we eliminate all environmental variance (VE = 0), then VGE = 0 (because, if either VG or VE is zero, no genetic - environmental interaction can take place), and VP = VG. In theory, we might make VE equal to 0 by ensuring that all individuals were raised in exactly the same environment but, in practice, it is virtually impossible. Instead, we could make VG equal to 0 by raising genetically identical individuals, causing VP to be equal to VE. In a typical experiment, we might raise cloned or highly inbred, identically homozygous individuals in a defined environment and measure their phenotypic variance to estimate VE. We could then raise a group of genetically variable individuals and measure their phenotypic variance (VP). Using VE calculated on the genetically identical individuals, we could obtain the genetic variance of the variable individuals by subtraction:

It is symbolized H2 because it is a measure of variance, which is in units squared.

Broad-sense heritability can potentially range from 0 to 1. A value of 0 indicates that none of the phenotypic variance results from differences in genotype and all of the differences in phenotype result from environmental variation. A value of 1 indicates that all of the phenotypic variance results from differences in genotype. A heritability value between 0 and 1 indicates that both genetic and environmental factors influence the phenotypic variance.

Often, we are more interested in the proportion of the phenotypic variance that results from the additive genetic

VG[of genetically varying individuals]

VP[of genetically varying individuals] VE[of genetically identical individuals]

The broad-sense heritability of the genetically variable individuals would then be calculated as follows:

VG[of genetically varying individuals]

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