Allele

An allele is one of the alternative inherited states at a genetic locus, often distinguished by a difference in DNA sequence. A population can contain many alleles, although a two-copy diploid individual carries at most two at that genetic locus, one inherited from each parent. See NHGRI: allele.

Count copies, not just individuals, when computing allele frequency: in 100 individuals with genotype counts

49 AA

42 A a

9 aa

, there are

2 (49) + 42 = 140

copies of

A

among 200, so

p_{A} = 0.70

. The heterozygotes contribute to both allele counts. This converts observed genotypes into the population quantity needed by the Hardy-Weinberg principle.

Table of contents
- Allele frequency Allele
  - Allele fixation Allele frequency
    - Fixation probability Allele fixation

Allele frequency ( $p_{A}$ )

 0  0

The allele frequency is the proportion of gene copies at a specified genetic locus carrying an allele. Two unrelated copies can have the same observed allele without being identical by descent; under independent sampling their matching probability is the sum of squared allele frequencies. Pedigree autozygosity instead traces copies to the same ancestral copy.

Allele fixation ( $u = 0 or u = 1$ )

 0  0

An allele fixes when all copies at the relevant locus in the population are that allele. Fixation can occur through selection or genetic drift. Spatial polymorphism means that fixation within an individual habitat need not imply fixation across a metapopulation.

Fixation probability ( $P_{fix}$ )

 0  0

The probability that an allele eventually occupies every copy at the specified genetic locus under a population model. For a rare favorable variant in a suitable large-population, small-effect branching approximation,