# Allele Frequency

Allele frequency or Gene frequency is the proportion of all copies of a gene that is made up of a particular gene variant (allele). In other words, it is the number of copies of a particular allele divided by the number of copies of all alleles at the genetic place (locus) in a population. It can be expressed for example as a percentage. In population genetics, allele frequencies are used to depict the amount of genetic diversity at the individual, population, and species level. It is also the relative proportion of all alleles of a gene that are of a designated type.

Given the following:

1. a particular locus on a chromosome and the gene occupying that locus
2. a population of N individuals carrying n loci in each of their somatic cells (e.g. two loci in the cells of diploid species, which contain two sets of chromosomes)
3. different alleles of the gene exist
4. one allele exists in a copies

then the allele frequency is the fraction or percentage of all the occurrences of that locus that is occupied by a given allele and the frequency of one of the alleles is a/(n*N).

For example, if the frequency of an allele is 20% in a given population, then among population members, one in five chromosomes will carry that allele. Four out of five will be occupied by other variant(s) of the gene.

Note that for diploid genes the fraction of individuals that carry this allele may be nearly two in five (36%). The reason for this is that if the allele distributes randomly, then the binomial theorem will apply: 32% of the population will be heterozygous for the allele (i.e. carry one copy of that allele and one copy of another in each somatic cell) and 4% will be homozygous (carrying two copies of the allele). Together, this means that 36% of diploid individuals would be expected to carry an allele that has a frequency of 20%. However, alleles distribute randomly only under certain assumptions, including the absence of selection. When these conditions apply, a population is said to be in Hardy–Weinberg equilibrium.

The frequencies of all the alleles of a given gene often are graphed together as an allele frequency distribution histogram, or allele frequency spectrum. Population genetics studies the different "forces" that might lead to changes in the distribution and frequencies of alleles—in other words, to evolution. Besides selection, these forces include genetic drift, mutation and migration.

### Other articles related to "allele frequency, allele, frequency, alleles":

Allele Frequency - The Effect of Mutation
... Let ú be the mutation rate from allele A to some other allele a (the probability that a copy of gene A will become a during the DNA replication ... If is the frequency of the A allele in generation t, then is the frequency of the a allele in generation t, and if there are no other causes of gene frequency change (no natural selection, for ... This tells us that the frequency of A decreases (and the frequency of a increases) by an amount that is proportional to the mutation rate ú and to the ...
Genome-wide Association Study - Methods
... of these SNPs it is then investigated if the allele frequency is significantly altered between the case and the control group ... of individuals in the case group having a specific allele, and the proportions of individuals in the control group having the same allele ... When the allele frequency in the case group is much higher than in the control group, the odds ratio will be higher than 1, and vice versa for lower allele frequency ...
Db SNP - Submission - 3. How To Submit
... Alleles Alleles must be defined using A, G, C, or T nomenclature IUPAC nomenclature will only be accepted in flanking regions ... DNA sequencing) or how the allele frequencies were calculated ... A description of the initial group from which the variation was found or from which the allele frequency was calculated ...
Genotype Frequency - Numerical Example
... When calculating an allele frequency for a diploid species, remember that homozygous individuals have two copies of an allele, whereas heterozygotes have only one ... In our example, each of the 42 pink-flowered heterozygotes has one copy of the a allele, and each of the 9 white-flowered homozygotes has two copies ... Therefore, the allele freguency for a (the white color allele) equals This result tells us that the allele frequency of a is 0.3 ...

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