How to Determine Allele Frequencies

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The genetic code allows the transfer of information from one generation to the next and is encoded within deoxyribonucleic acid. A gene is a functional unit of heredity and normally codes for the production of a specific protein. An allele is specific form of a gene and can be dominant or recessive. For example, there are different alleles for blood type. Allele frequency is a measure of the relative frequency of different alleles within a population and can be calculated with ease.

Determining Allele Frequency

    Begin by making a table that lists possible combinations of alleles and how many there are of each type. Let's look at human blood type alleles in a sample population. Assume that two alleles are possible for blood type, one that codes for a protein A and one that codes for a protein B. These alleles combine to give the genotype of the individual, which can be AA, AB or BB. Doctors and scientists can use simple testing to determine which genotype an individual has. In our example, the blood type alleles and counts of 950 people look like the following: AA = 250 AB = 550 BB= 150

    Next, find the total allele count. This can be found by adding the count for all of the blood type alleles and multiplying by 2. In this example, the total allele count is: 2 x (250 + 550 + 150) = 1900

    In order to determine the allele frequency, choose an allele, in this case A or B, count the number of that type of allele and then divide this by the total allele count. Following the example, there are two of the allele A in the genotype AA but only one of the allele A in the genotype AB, so: Allele frequency of A = ( (2 x 250) + 550 ) / 1900 = 0.55

    In a similar way, there are two of the allele B in BB but only one in AB, so: Allele frequency of B = ( (2 x 150) + 550) / 1900 = 0.45


    • In addition to A and B, humans have another blood type allele, O. Type O alleles do not code for a protein.


About the Author

Samuel Markings has been writing for scientific publications for more than 10 years, and has published articles in journals such as "Nature." He is an expert in solid-state physics, and during the day is a researcher at a Russell Group U.K. university.

Photo Credits

  • Tomasz Wyszołmirski/iStock/Getty Images

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