Allelic Frequency: Definition & Explanation

Instructor: Katie Chamberlain

Katie has a PhD in Microbiology and has experience preparing online education content in Biology and Earth Science.

Allelic frequency is a calculation used in genetics. This lesson will recap some basic genetics and go over allelic frequency. Test your genetics and math skills with the quiz.

What Is Allelic Frequency?

Allelic frequency is the frequency of an allele in a population. It is calculated by counting the number of times an allele occurs in a population and dividing by the total number of occurrences of all alleles for that gene.

What Is an Allele?

Let us review some basic genetics. Genes are segments of DNA that encode recipes for different products, usually proteins. A person's genes contain the blueprint for building and running their entire body. Everything from height, hair color, eye color, ability to process sugar, shape of blood cells, etc. is contained in genes. However, since not everyone is the same, we know that not all blueprints are exactly the same. In other words, people must have different versions of genes.

The different versions of genes are called alleles. Some genes only come in one allele, some come in two alleles, but many genes have multiple different alleles. Let's imagine a flower. The color of the petals is determined by a single gene for petal color and each flower has one version of this gene. If a flower has the allele for red, then it has red petals. If it has the allele for white, then it has white petals. White and red are alleles of the petal color gene.

Allelic Frequency

In a population there are many individuals, and each individual has many different genes. Each of these genes could come in different alleles. When scientists study the genetics of a population, one of the things they look at is the prevalence of different alleles.

Let's consider our flower example. Imagine a field of 98 red flowers and 2 white flowers. A regular person counting the flowers would say that 98% of the flowers were red and 2% of the flowers were white. A geneticist would say that the allelic frequency was 98% red and 2% white.

Allelic Frequency and Diploids

In our flower example, it was easy to count the allelic frequency because the flowers each only contained one allele. The red flowers contained a red allele and the white flowers contained a white allele. But, what if the flowers were diploid, (like humans)? This would mean that they each have two copies of every gene.

They might have two copies of the same allele, called a homozygote. Or, their two copies might be different, a heterozygote. For heterozygotes, the allele that is expressed in the phenotype (physical appearance) is determined by dominance. There are many types of dominance, but that is for another lesson.

If we pretend that red was the straight-up dominant petal color, the red flowers could be either homozygotes (red, red) or heterozygotes (red, white). This would cause a change in the allelic frequency we calculated above. If the red flowers could have a white allele, then the white allelic frequency would be much higher than we thought (and the red allelic frequency would be lower).

Let's pretend that we collected DNA and looked at both alleles for petal color in all 100 diploid flowers. We found that 48 red flowers were heterozygous (48 red alleles and 48 white alleles), 50 red flowers were homozygous red (100 red alleles), and 2 were homozygous white (4 white alleles). There would be 48+100 = 148 red alleles. There would be 48+4 = 52 white alleles. The allelic frequencies would be 148/200 = 74% red and 52/200 = 26% white.

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