Polygenic inheritance

Abstract

The association between the Mendelian inheritance of alleles and changes in allele frequencies in populations (population genetics) is obvious, as described in Chapter 3. The relationship between allelic inheritance and the inheritance of complex traits like growth, maturity, behavior etc. (quantitative genetics) is often less apparent. Population genetics deals with allelic variation, which may often be detected by chemical analysis at the level of protein or DNA polymorphism. The effects of the alleles on the phenotype is usually unknown, or mainly of interest as markers of allelic variation. The analysis of the inheritance of complex traits, usually termed quantitative genetics, deals with phenotypic measurements and observations, and the underlying allelic constitution is normally unknown. Nevertheless, quantitative genetics is also based on the Mendelian laws of inheritance. A simple example of the relationship between Mendelian genetics, population genetics and quantitative genetics is shown in Table 4.1. A gene occurs in a population as two different alleles, a and A, both with an allele frequency of 0.5. At Hardy-Weinberg equilibrium (Chapter 3.2), this will result in a distribution of allelic genotypes as shown in the table. If the alleles have an effect on a phenotypic trait, e.g. that A has an additive effect of +1 unit of measurement compared to a, the genetic effects of the different allelic genotypes on the phenotypes will be as shown in the table.