Population genetics · BEGINNER LESSON
Allele frequency
Allele frequency is the proportion of copies of a specific genetic variant, called an allele, in a population. It is used to describe how common that allele is and to compare observed rarity or commonness with disease patterns.
What it is
An allele is one version of a gene. Allele frequency is the share of all gene copies in a population that are that version. In a two-allele case, the frequencies add to 1, so knowing one allele’s frequency tells you the other one. In finite populations, these frequencies can change from generation to generation because adults do not all contribute equally to the next generation.
How it is used
In medical genetics, comparing a variant’s allele frequency with the rarity of a disorder can help judge whether the variant is likely to cause disease. Public population databases are used for this comparison. Some guidelines use thresholds such as BS1 or BA1 for commonness, while PM2 covers rarity; these thresholds are tied to disease prevalence, inheritance pattern, and penetrance, which is the proportion of people with a variant who show the disease.
[1] Assessing Allele Frequency Information: A Study of Variant ...
How frequency changes and why interpretation is limited
The Wright-Fisher model describes random genetic drift, meaning chance fluctuations in allele frequency in finite populations. It is a discrete-time model, with each step representing a generation. Its assumptions include weak or multiplicative selection, and these assumptions shape how well the model fits real data. For clinical interpretation, allele frequencies can vary by ancestry or other population structure, so reference groups must be well characterized. Rarity alone is not enough to prove pathogenicity, and very rare private variants may be absent from a population sample even when they are real.
[1] Assessing Allele Frequency Information: A Study of Variant ... [2] Methods [3] Advancing Risk Gene Discovery Across the Allele Frequency ... - NIH
SEE THE IDEA
From population count to interpretation
Count allele copies in a defined group of people.
Divide copies of the allele by total allele copies to get its proportion.
Compare the frequency with expected rarity from prevalence, penetrance, and inheritance.
Check ancestry, sample quality, and whether rarity alone is being overread.
The measured frequency feeds into disease comparison, but population context limits what the number can prove.
Allele frequency is a population measurement first; interpretation comes only after comparing it with disease expectations and sample context.
TRY IT YOURSELF
Test your understanding
Choose an answer, then check the explanation.
PUT IT TOGETHER
Match each concept to its meaning
Drag a label onto its description, or select a label and then select a description. This also works with a keyboard.
4 pairs to match
Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.