# How to determine allele frequencies

Written by tiffanyc
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An allele is one version of a specific gene. There may be different versions of the gene due to changes in the DNA sequence of the gene. Humans usually have two alleles in one gene; one copy from each parent. The number of times an allele appears in a population is the allele frequency, which is used to show genetic variation involved in a population. To determine whether the allele frequency of an ideal population does not change over time, apply the Hardy-Weinberg equation.

Skill level:
Moderate

## Instructions

1. 1

Find out how many copies of the specific allele of interest are in the population.

2. 2

Find out the total number of alleles, including the allele of interest, that are in the population.

3. 3

To determine the allele frequency, divide the total number of alleles in a population (Step 2) by the number of alleles of interest (Step 1).

1. 1

Assume there are two alleles of a gene in the population. Let the first allele of a gene in a population be denoted as 'W' and the second allele 'w'.

2. 2

Find the allele frequency of each allele.

3. 3

Since there are two alleles in the population, adding the frequencies of the alleles together should equal to one.

W + w = 1.

Therefore, if you were to find out the frequency of one of the alleles, for instance, w, simply subtract W from 1.

w = 1 - W

4. 4

Calculate the genotypic frequency of the allele distribution in the population using the Hardy-Weinberg equation. Since there are two alleles in a gene, the possibilities of an offspring receiving the combination of alleles from their parents are: WW, Ww, wW and ww. This represents the genotype of an individual. Since Ww is the same as wW, there are two chances that the individual will receive this genotype, hence you can group it as '2Ww'.

#### Tips and warnings

• Hardy-Weinberg states that the number of genotypic frequencies should equal to one, in the following relation: W^2 + 2Ww + w^2 = 1. The reason why 'W' and 'w' are squared is due to the instances when the allele is shown twice in the genotype (WW or ww), raising the allele to the power of 2. To determine a specific genotype (for example, ww), simply use the allele frequency equation (w = 1-W) to insert the genotypic frequency of ww in the equation.
• The Hardy-Weinberg equation is only used to determine if the allele frequency of the population does not change. The Hardy-Weinberg equation does not work if the population undergoes selection, mutation, migration, genetic drift or random mating.

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