Quantitative Genetics and Quantitative Traits

Introduction In the chapter on Mendel and Morgan, we saw how the transmission of genes from one generation to another follows precise mathematical formula. The traits discussed in that chapter, however, were discrete traits—peas are either yellow or green, someone either has a disorder or does not have the disorder. But many behavioral traits are not like these clear-cut, have-it-or-don’t-have-it phenotypes. People vary from being quite shy to very outgoing. But is shyness a discrete trait or merely a descriptive adjective for one end of a continuous distribution? In this chapter, we will discuss the genetics of quantitative, continuously distributed distribution. Let us note first that genetics has made important—albeit not well recognized—contributions to quantitative methodology in the social sciences. The concept of regression was initially developed by Sir Francis Galton in his attempt to predict offspring’s phenotypes from parental phenotypes; it was later expanded and systematized by his colleague, Karl Pearson, in the context of evolutionary theory. The analysis of variance was formulated by Sir Ronald A. Fisher to solve genetic problems in agriculture. Finally, the famous American geneticist Sewell Wright developed the technique of path analysis which is now used widely in psychology, sociology, anthropology, and other social sciences.