By Adrianne Jerrett, Updated Aug 30, 2022
Deoxyribonucleic acid (DNA) is the molecule that carries the genetic code for nearly all living organisms on Earth. Its variations underpin the unique traits—eye color, skin tone, height—that distinguish individuals within a species.
DNA is organized into genes, each positioned at a specific locus on a chromosome. A single gene can exist in multiple forms—alleles—which arise from different nucleotide sequences.
Alleles directly shape observable traits, or phenotypes. For instance, variations in the eye‑color gene produce blue, green, brown, and hazel phenotypes. Individuals with blue eyes carry a distinct allele sequence from those with brown, hazel, or green eyes.
The allele frequency refers to the proportion of a specific allele within a population. By quantifying allele frequencies, researchers can gauge the prevalence of particular phenotypes and monitor genetic diversity over time.
1. Count the total number of individuals in the population.
2. Determine how many individuals display each phenotype of interest.
3. Multiply the number of individuals by the number of allele copies per individual (usually two for diploid organisms).
4. Divide the total count of a specific allele by the total number of allele copies in the population.
Assume a sample of 100 people with two eye‑color alleles: B (blue) and G (green). Each person carries two alleles, so the population contains 200 allele copies.
Genotype distribution:
• BB: 50 individuals
• BG: 23 individuals
• GG: 27 individuals
Genotypic frequencies:
• BB: 50/100 = 0.50 (50%)
• BG: 23/100 = 0.23 (23%)
• GG: 27/100 = 0.27 (27%)
Allele counts:
• B alleles = (50 × 2) + 23 = 123
• G alleles = (27 × 2) + 23 = 77
Allele frequencies:
• B = 123/200 = 0.615 (61.5%)
• G = 77/200 = 0.385 (38.5%)
As a sanity check, the allele frequencies should sum to 1 (or 100%). Here, 61.5% + 38.5% = 100%.
These calculations reveal that the B allele is more common in this population, indicating a higher prevalence of the blue phenotype. Tracking these frequencies across successive generations can uncover shifts driven by natural selection, migration, or genetic drift, offering insight into the evolutionary dynamics of the group.
