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Modern Genetic Analysis Chapter 18. Quantitative Genetics
Summary

Manyperhaps mostof the phenotypic traits we observe in organisms vary continuously. In many cases, the variation of the trait is determined by more than a single segregating locus. Each of these loci may contribute equally to a particular phenotype, but it is more likely that they contribute unequally. The measurement of these phenotypes and the determination of the contributions of specific alleles to the distribution must be made on a statistical basis in these cases. Some of these variations of phenotype (such as height in some plants) may show a normal distribution around a mean value; others (such as seed weight in some plants) will illustrate a skewed distribution around a mean value.

In other characters, the variation in one phenotype may be correlated with the variation in another. A correlation coefficient may be calculated for these two variables.

A quantitative character is one for which the average phenotypic differences between genotypes are small compared with the variation between the individuals within the genotypes. This situation may be true even for characters that are influenced by alleles at one locus. The distribution of environments is reflected biologically as a distribution of phenotypes. The transformation of environmental distribution into phenotypic distribution is determined by the norm of reaction. Norms of reaction can be characterized in organisms in which large numbers of genetically identical individuals can be produced.

With the use of genetically marked chromosomes, it is possible to determine the relative contributions of different chromosomes to variation in a quantitative trait, to observe dominance and epistasis from whole chromosomes, and, in some cases, to map genes that are segregating for a trait.

Traits are familial if they are common to members of the same family, for whatever reason. Traits are heritable, however, only if the similarity arises from common genotypes. In experimental organisms, environmental similarities may be readily distinguished from genetic similarities, or heritability. In humans, however, it is very difficult to determine whether a particular trait is heritable. Norm of reaction studies show only small differences between genotypes, and these differences are not consistent over a wide range of environments. Thus, "superior" genotypes in domesticated animals and cultivated plants may be superior only in certain environments. If it should turn out that humans exhibit genetic variation for various mental and emotional traits, this variation is unlikely to favor one genotype over another across a range of environments.

The attempt to quantify the influence of genes on a particular trait has led to the determination of heritability in the broad sense (H2). In general, the heritability of a trait is different in each population and each set of environments and cannot be extrapolated from one population and set of environments to another. Because H2 characterizes present populations in present environments only, it is fundamentally flawed as a predictive device. Heritability in the narrow sense, h2, measures the proportion of phenotypic variation that results from substituting one allele for another. This quantity, if large, predicts that selection for a trait will succeed rapidly. If h2 is small, special forms of selection are required.

© 1999 by W. H. Freeman and Company. All rights reserved.