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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

n = k2+k/2, where k = # alleles at a specific locus

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:


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

# Individuals with each genotype

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

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