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Allele Frequency – Definition and Medical Relevance

Allele frequency describes how often a specific variant of a gene occurs in a population. It is a key measure in population genetics and medical genomics.

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Things worth knowing about "Allele Frequency"

Allele frequency describes how often a specific variant of a gene occurs in a population. It is a key measure in population genetics and medical genomics.

What Is Allele Frequency?

Allele frequency refers to how often a particular version of a gene – known as an allele – appears in a given population. It is expressed as a proportion of all gene copies at a specific genomic location, called a locus, and ranges from 0 to 1 (or 0% to 100%).

Since most humans carry two copies of each gene (one inherited from each parent), a population of N individuals contains 2N alleles at any given locus. The allele frequency is calculated by dividing the count of a specific allele by the total number of alleles at that locus across the entire population.

Foundations in Population Genetics

Allele frequency is a cornerstone concept of population genetics – the branch of genetics that studies the genetic composition of populations and how it changes over time. The Hardy-Weinberg equilibrium describes the theoretical state in which allele frequencies remain constant across generations, assuming no evolutionary forces are acting on the population.

Factors That Change Allele Frequencies

Natural selection: Alleles that increase survival or reproductive success become more common over generations.
Genetic drift: Random fluctuations in small populations can cause allele frequencies to change by chance alone.
Mutation: New alleles arise through changes in the DNA sequence.
Gene flow (migration): Movement of individuals between populations introduces or removes alleles.
Non-random mating: Preference for certain traits influences which alleles are more frequently combined.

Clinical Relevance

In medicine, allele frequency plays an important role in assessing the risk of genetic diseases within specific populations. When a disease-associated allele is particularly common in a population, the likelihood that individuals from that group will be affected or will pass on the condition increases. Well-known examples include:

Sickle cell disease: The causative allele is especially frequent in certain African populations due to a selective advantage against malaria.
Cystic fibrosis: The most common disease-causing allele (ΔF508) is predominantly found in European populations.
Lactose intolerance: The persistence of the lactase allele varies significantly across populations worldwide.

Calculating Allele Frequency

Allele frequency is calculated using the following formula:

Allele frequency = Number of copies of the allele / Total number of alleles at that locus

Example: In a population of 100 individuals, 20 carry allele A on both chromosomes (homozygous) and 50 carry it on one chromosome (heterozygous). This gives 20 × 2 + 50 × 1 = 90 copies of allele A out of 200 total alleles. The allele frequency of A is therefore 90 / 200 = 0.45 (45%).

Importance in Modern Genomics

Modern research tools such as Genome-Wide Association Studies (GWAS) allow scientists to identify alleles associated with specific diseases or traits. Databases such as gnomAD and the 1000 Genomes Project catalog the allele frequencies of thousands of genetic variants across diverse global populations. These resources help distinguish rare from common variants and improve our understanding of the genetic basis of complex diseases.

References

Strachan T, Read A. Human Molecular Genetics. 4th ed. Garland Science; 2011.
Hartl DL, Clark AG. Principles of Population Genetics. 4th ed. Sinauer Associates; 2007.
Karczewski KJ et al. The mutational constraint spectrum quantified from variation in 141,456 humans. Nature. 2020;581:434-443.

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