Chapter 18 - Population and Evolutionary Genetics

Abstract

Population genetics is the quantitative study of the distribution of genetic variation in a population and of how the frequencies of its genotypes, alleles, and phenotypes are maintained or changed. It seeks answers to such practical questions as why the frequency of PKU in Caucasians is so much greater than in Japanese, or why the frequency of the sickle cell allele varies markedly in people from different West African countries. The mathematical cornerstone of population genetics is the Hardy-Weinberg law or principle. The law has two parts. First, it states that in a large, randomly mating population with two alleles at a locus (for example, A and a), there is a simple relationship between these allele frequencies (frequency of A = p; frequency of a = q) and the genotype frequencies (p 2 , 2pq, or q 2 ) they define. Second, it holds that this relationship between allele and genotype frequencies, constructed simply on the binomial expansion of (p + q) 2 , does not change from one generation to the next. When a population conforms to this two-part law, it is in Hardy-Weinberg equilibrium. In such populations, the law is of great value in showing why dominant traits do not increase in frequency from one generation to the next and why recessive traits do not decrease. Further, the law is regularly used in genetic counseling settings where estimates of genotype, allele, and carrier frequencies are calculated from limited phenotypic information in small families, such estimates then being employed to estimate specific genetic risk.