20.7 Quantifying heritability 657 environment promoting musical performance. The result would be a greater variance among people in musical ability than would be the case if there were no effect of the parental environment on children. If the phenotype is the sum of a genetic and an environmental effect, p 5 g 1 e , then the variance of the phenotype is, accord-ing to the formula on page 672, the sum of the genetic variance, the environmental variance, and twice the covariance between the genotypic and environmental effects: If genotypes are not distributed randomly across envi-ronments but this is not taken into account, there will be some covariance between genotypic and environmen-tal values, and that covariance will be hidden in the genetic and environmental variances. The quantitative measure of heritability of a charac-ter is that part of the total phenotypic variance that is due to genetic variance: H 2 , so deFned, is called the broad heritability of the character. It must be stressed that this measure of “genetic in-±uence” tells us what part of the population’s variation in phenotype can be attributed to variation in genotype. It does not tell us what parts of an individual’s pheno-type can be ascribed to its genotype and to its environ-ment. This latter distinction is not a reasonable one. An individual’s phenotype is a consequence of the interac-
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.