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How to find allele frequency in a gene pool

906 views · Published 25 August 2014 · 9:11 · Indexed 20 October 2026

Channel: Nikolay's Genetics Lessons · 2014 · Education

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Deriving genotypic and allelic frequencies

One definition of a population is its genotypic frequencies. This gives an indication of the genotypes which are the most or least prevalent in the population. The examples that will be used are for the blood type locus. Two alleles are possible, M or N, that can combine to give the following genotypes MM, MN, NN. The following data was collected from a human population.
Genotype # of Individuals Genotypic frequencies
MM 1787 MM = 1787/6129 = 29
MN 3039 MN = 3039/6129 = 50%
NN 1303 NN = 1303/6129 = 21%
Total 6129 
The population that we just analyzed has just two alleles at the blood type locus, and each individual will have some combination of these two alleles. Thus, this entire population consists of 12,258 (6129 X 2) alleles at the blood type locus. To deter mine the allelic frequencies we simply count the number of M or N alleles and divide by the total number of alleles. So the allelic frequency for the M allele will be:

f(M) = [(2 x 1787) + 3039]/12,258 = 0.5395

and the frequency for the N allele will be:

f(N) = [(2 x 1303) + 3039]/12,258 = 0.4605

By convention one of the alleles is given the designation P and the other q. Thus for the data we presented above, p=0.5395 and q=0.4605. Because we are analyzing all the alleles, the frequencies shoul d sum to 1.0 and p + q = 1. Furthermore, a population is considered by population geneticists to be polymorphic if two alleles are segregating and if the frequency of the most frequent allele is less than 0.99.

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