How To Determine Allele Frequencies

Deoxyribonucleic acid, more popularly known as DNA, is a molecule found in the large majority of living organisms and viruses found on Earth. DNA carries the genetic information, or the code, that makes everything what it is.

DNA differs between species and between individuals within a species. For example, in humans, DNA determines the color of human eyes, skin, hair, height and every other attribute that makes each person unique.

DNA is made up of different genes. Genes carry the genetic information from each parent.

Each gene is found on a specific locus on a chromosome. A gene may have multiple variations made up from different allele sequences.

Alleles determine visible individual characteristics, called phenotypes. For example, blue, green, brown and hazel are all different phenotypes for the human eye.

When looking at one of the many genes for eye color across a group of people, those with blue eyes would have a different sequence of alleles than those with brown, hazel and green eyes.

The allele frequency is the number of individuals in a population that have a specific allele type. People use the allele frequency calculation to help understand the rate a phenotype occurs in a population.

This information gives an understanding of the genetic diversity in a population. When allele frequencies are recorded over time, changes in genetic diversity can be observed.

To calculate allele frequency, the total number of individuals in the population must be counted. Then, count the number of individuals that each has the specific phenotype in question.

Create a tally of all the totals. To find the allele frequencies divide the number of times an allele is counted in a population by the total number of allele copies found in that gene.

For example, say there are 100 individuals in a population and two types of alleles, B for blue eyes and G for green eyes. Each person has two copies of each allele so multiply two by 100 to give 200 allele copies in the population.

In real life, there are many genes that code for human eye color, but for this scenario, there are only three different allele combinations in this gene pool; BB, BG and GG. Next count the number of people in the population with each allele type.

In this example, there are 50 people with BB, 23 people with BG and 27 people with GG. To find the genotypic frequencies simply divide the number of people with a specific phenotype by the total number of people.

In this case, 50 BB divided by 100 people means that 50 percent of the population has the BB genotype. The gene frequency for BG would be 23 percent and 27 percent of people in the gene pool would have the GG gene type.

While genotypic frequencies look at the expression of the genes, allele frequencies look at the number of times a specific allele occurs in a population. To find the allele frequency of B in this example multiply 50 by two as there are two B's in the BB genotype.

Then add the people with the BG genotype as they also each have a B allele, giving a total of 123 B alleles. Finally, divide 123 by 200 as each person in the population carries two alleles, giving an allele frequency of 0.615 or 61.5 percent.

Next, do the same for the G allele. By multiplying the 27 people with GG alleles by two and adding the 23 people who also have a G allele then dividing this number, 77, by 200, results in 0.385 or 38.5 percent.

Check for mistakes by ensuring that all the allele frequencies add up to 1 or 100 percent. Here, 61.5 added to 38.5 equals 100.

These calculations have provided the information as to how many people have blue phenotype eyes and how many have green phenotype eyes in this population of 100 people. From the allele frequencies, it is evident that the B allele is more dominant in a population.

By continuing this study in future generations, it will become apparent if there are changes in the allele frequencies over time and provide some insight into the evolution of the population.