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Calculating Allele Frequencies

Microevolution is a change in allele frequencies from one generation to the next.

Allele frequency is the relative proportion of different alleles for a given gene that within a population. It is a measure of the relative prevalence of a particular allele in a gene pool. A gene pool is the total set of genes of all individuals in a population. Microevolution occurs when there is a shift in the proportion of alleles in a gene pool. Specifically, microevolution is a change in allele frequencies within a population. Frequencies are numbers that run from zero to one (a proportion), while percentages run from 0% to 100%.

Genotype frequency is the proportion of individuals in a population that have a particular genotype, or genetic makeup. To calculate allele frequencies, it is often easiest to start with genotype frequencies. Because genotype frequencies are proportions, they always add up to 1. For example, cystic fibrosis is a human condition caused by having two copies of the cystic fibrosis allele (c) at a single locus. The unaffected allele in this example is C. Imagine a population of 10 individuals. Seven people are homozygous (having two identical alleles for a gene) for the unaffected allele. So they have the genotype CC. Two people are carriers of the cystic fibrosis allele. They are heterozygous, meaning they have one dominant allele and one recessive allele, Cc. And one person is homozygous for the cystic fibrosis allele. This person has the genotype cc. The genotype frequencies can then be calculated.
Genotype frequencies of cystic fibrosis in a population can be calculated. To do so, take the number of people with each genotype (CC, Cc, cc) and divide it by the total population. Multiple this number by 100 and that is the percentage of people with that genetic condition.
The allele frequencies are calculated slightly differently because there are two slots for possible alleles at each locus. Returning to the example of cystic fibrosis, the total number of slots is equal to twice the total number of individuals in the population (10 individuals × 2 = 20 slots for alleles). Homozygous individuals count twice and heterozygous individuals count once in these calculations. The allele frequencies can be calculated as follows:
C=((2×7)+(1×2))/20=0.80or80%\mathrm C\;=\;((2\times7)+(1\times2))/20=0.80\;\mathrm{or}\;80\%
c=((2×1)+(1×2))/20=0.20or20%\mathrm c\;=((2\times1)+(1\times2))/20=0.20\;\mathrm{or}\;20\%
As with genotype frequencies, allele frequencies must equal one or 100%.