Aa Bb

0123456789 10 Phenotypic value (units of pigment)

Figure 9.4 Phenotypic distributions for a trait determined by two loci in populations with low additive genetic variation (VA). Genetic variation in phenotypes is additive since each a or b allele in a genotype contributes 1/4 unit of pigment while each A or B allele in a genotype contributes 21/4 units of pigment irrespective of genotype. However, total genetic variation is relatively low since allele frequencies are near fixation and loss (the frequency of a and b alleles is 0.1 while the frequency of A and B alleles 0.9). Compare with Fig. 9.2 where allele frequencies are all equal to 1/2 and allelic effects are additive.

individuals and phenotypic variation caused by the different environments individuals experience.

Quantifying and comparing the variance of quantitative traits utilizes a set of summary statistics. Since quantitative trait distributions usually approximate hump-shaped normal distributions, statistics that describe normal distributions are useful (see the Appendix for a primer on basic statistics used in quantitative genetics). The middle or central tendency of a quantitative trait distribution is described by its average or mean. The spread of the observations around this central tendency is described by the variance of the distribution. The coefficient of var variation or CV (CV = ——

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