Allele Frequency and Genotype Frequency

Allele frequency is the proportion of a particular allele among all alleles at a locus in a population, and genotype frequency is the proportion of individuals carrying a particular combination of alleles. Together these two quantities are the basic descriptive measures of a population's genetic composition and the variables that every evolutionary force acts upon.

Definition

Allele frequency is the relative proportion of a given allele among all copies at a locus in a population; genotype frequency is the proportion of individuals in the population that carry a specified genotype at that locus.

Scope

The entry defines the two frequency measures, shows how each is counted from genotype data, and explains the relationship between them under random mating. It also notes how allele-frequency data are organised in population reference databases. It is a conceptual and methodological topic, not clinical guidance.

Core questions

• How are allele frequencies counted from observed genotype counts?
• How are allele and genotype frequencies related under random mating?
• How do population reference databases report allele frequencies, and why do they differ between populations?

Key concepts

• Allele frequency
• Genotype frequency
• Allele counting from genotypes
• Minor allele frequency
• Polymorphism versus rare variant
• Population-specific reference frequencies

Mechanisms

Allele frequencies are obtained by counting allele copies across all sampled individuals and dividing by the total number of copies; for a biallelic locus the two allele frequencies sum to one. Genotype frequencies are the proportions of each genotype among individuals. Under random mating these two sets of numbers are linked by the Hardy-Weinberg relationship, so that genotype frequencies can be predicted from allele frequencies, and a sample's allele frequencies can be recovered from its genotype counts. Evolutionary forces change a population's composition precisely by altering these frequencies.

Clinical relevance

Population allele frequencies are central to interpreting genetic variants: a variant that is common in a matched reference population is generally less likely to be a cause of a rare disease, and carrier-frequency estimates derive directly from allele frequencies. These measures describe how variation is catalogued and weighed as evidence; they are not by themselves a basis for individual diagnosis or treatment.

Epidemiology

Allele frequencies vary among human populations as a result of their demographic histories, so large catalogues such as the 1000 Genomes Project report frequencies stratified by population to support population-matched interpretation of variation.

Evidence & guidelines

Population reference resources such as the 1000 Genomes Project provide allele-frequency data across many populations that are widely used as a baseline for assessing the rarity of genetic variants.

History

Counting alleles and genotypes became meaningful once Mendelian inheritance was framed in population terms in the early twentieth century. The Hardy-Weinberg principle made explicit how genotype frequencies relate to allele frequencies, and the later development of molecular markers and, ultimately, large sequencing consortia turned allele-frequency estimation into a routine, population-scale measurement.

Key figures

• G. H. Hardy
• Wilhelm Weinberg
• Sewall Wright

Seminal works

• hardy-1908
• 1000genomes-2015

Frequently asked questions

What is the difference between allele frequency and genotype frequency?

Allele frequency counts copies of an allele relative to all alleles at a locus, while genotype frequency counts individuals carrying a particular allele combination. Under random mating the two are linked by the Hardy-Weinberg relationship.

Why do allele frequencies differ between populations?

Because populations have different demographic histories — founder effects, bottlenecks, drift, migration, and local selection — that shape which alleles became common or rare, which is why reference databases report frequencies separately by population.