173: Population Genetics

Key Points

Application of population genetic principles is the key to the provision of traditional medical genetic services as it plays a role in

• Determination of frequency of genetic diseases

• Heterozygote carrier frequency estimation

• Genetic disease risk prediction

• Public health/epidemiology

• Role of genes versus environment

• Study of complex traits

Number of Genotypes at a Single Locus

The number of genotypes at a locus in a population is defined by

Allele Frequencies

Actual allele frequencies in a population sample can be determined by the counting method. First, perform genotype studies on a randomly selected group of unrelated individuals, as shown in the example below using short tandem repeat (STR) genotype data:

Example:

• STR genotype data (N = 129 individuals)Allele designations “14-20” refer to # of STRs at a particular locus

From the data above, the frequency of the “14” allele in this sample = 75/258, and that of the “17” allele = 72/258.

Standard Formula Notation Used in Population Genetics

• f (A) = p

• f (a) = q

• f (A/a) = 2pqThe sum total of all alleles at a locus = 1

• p + q = 1Sum total of all genotypes in a population = 1

• (p + q)² = 1

• (p + q)² = p² + 2pq + q² = 1, where

• p² = f (AA)

• 2pq = f (Aa)

• q² = f (aa)

• p + q + r +...+ n = 1 (sum of all allele frequencies adds to unity)

• (p + q + r +....+ n)² = 1 (sum of all genotype frequencies adds to unity)

• Genotype frequencies stabilize after one generation of random mating and remain constant with several assumptions (large population size, random mating, no mutation, selection, migration)

  • Change in allele or genotypic frequencies can occur if above assumptions deviate widely.

Calculation of Allele Frequencies

At a ...