Population Genetics I. What is Population Genetics

Gene Frequencies & Allele Frequencies
• Gene frequency refers to proportion of particular allelic
form among all copies of gene in population
• Usually estimated by sampling population
– diploid: 2 copies of gene
• homozygotes: 2 copies of allele
• heterozygotes: 1copy of each allele
– haploid: 1 copy of allele
• For two alleles, p + q = 1, where p and q are frequencies
of the two alleles
Calculating Genotype Frequencies
Relative frequencies of genotypes – proportion of organisms
that have the particular genotype
• The proportion of individuals
in a population with a
particular genotype
• fA/A =
• fA/a =
• fa/a =
N
f
A/A
40
0.40
A/a
47
0.47
a/a
13
0.13
Calculating allele frequencies
• If fA/A, and fa/a are the proportions of the
three genotypes at a locus with two
alleles, then the frequency p(A) of the A
allele and the frequency q(a) of the a allele
are obtained by counting alleles:
• p = fA/A + ½ fA/a
• q = fa/a + ½ fA/a
• p + q = fA/A + fa/a + fA/a = 1.00
• q = 1 – p and p = 1 – q
2