- Allele frequency refers to how common a particular allele is within a population. Every gene can have multiple alleles, and the frequency of each allele reflects its representation in the population’s gene pool. Because populations evolve over generations, allele frequencies are not fixed—they shift due to random events, environmental pressures, and reproductive patterns. These shifts form the foundation of evolutionary change.
- In diploid organisms, each individual carries two alleles per gene, so allele frequency is calculated by counting all copies of an allele and dividing by the total number of alleles in the population. For example, in a population of 100 individuals, there are 200 total alleles for a gene. If 60 of those alleles are “A,” then the allele frequency of A is 0.30 (30%). This simple measure allows biologists to track how traits spread, decline, or remain stable across generations.
- Changes in allele frequency occur through several evolutionary mechanisms. Genetic drift causes random fluctuations, especially in small populations, sometimes leading to fixation or loss of alleles. Natural selection increases the frequency of alleles that improve survival or reproduction. Mutation introduces new alleles, often at very low initial frequencies. Gene flow—movement of individuals between populations—adds or removes alleles, altering local frequencies. Together, these forces determine the genetic structure of populations and drive evolutionary trajectories.
- Allele frequency is also central to understanding phenomena such as population bottlenecks, founder effects, inbreeding, and adaptation. In conservation biology, monitoring allele frequencies helps assess genetic diversity and population health. In medical genetics, allele frequencies reveal how common disease‑associated variants are in different human populations. In evolutionary biology, they provide quantitative evidence of how species change over time.
