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Figure 9.18 Distributions of estimated OTL effect sizes from several animal species. The distributions show the difference in phenotypic values for alternative homozygous genotypes (or the equivalent of 2 a) at a OTL in units of phenotypic standard deviations. OTLs with larger effect sizes explain a large proportion of the additive variance in a quantitative trait. The distribution in (a) represents OTLs identified from multiple phenotypes in dairy cattle while the distribution in (b) represents OTLs identified in multiple phenotypes in pigs. The distribution in (c) shows OTLs located on the third chromosome that influence sternopleural bristle number in D. melanogaster. Data in (a) and (b) from Hayes and Goddard (2001); data in (c) from Shrimpton and Robertson (1988).

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 QTL effect size (phenotypic standard deviations)

Figure 9.18 Distributions of estimated OTL effect sizes from several animal species. The distributions show the difference in phenotypic values for alternative homozygous genotypes (or the equivalent of 2 a) at a OTL in units of phenotypic standard deviations. OTLs with larger effect sizes explain a large proportion of the additive variance in a quantitative trait. The distribution in (a) represents OTLs identified from multiple phenotypes in dairy cattle while the distribution in (b) represents OTLs identified in multiple phenotypes in pigs. The distribution in (c) shows OTLs located on the third chromosome that influence sternopleural bristle number in D. melanogaster. Data in (a) and (b) from Hayes and Goddard (2001); data in (c) from Shrimpton and Robertson (1988).

tion closer to the optimum. Orr's model differs from Fisher's because it predicts that OTLs with larger effects can be fixed by selection. In addition, Orr's model suggests that the distribution of OTL effect sizes should have an increasing number of OTLs with small effects so that the number of OTLs increases rapidly as the OTL effect size decreases. Looking at the data provided

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