Allele frequency is a measure of the relative frequency of an allele on a genetic locus in a population. Usually it is expressed as a proportion or a percentage. In population genetics, the term is used to express the genetic diversity of a species population, or equivalently the richness of its gene pool. Allele frequency is defined as follows: An allele is any one of a number of viable DNA codings of the same gene (sometimes the term refers to a non-gene sequence) occupying a given locus (position) on a chromosome. ... Proportion A proportion is an equation with a ratio on each side. ... A percentage is a way of expressing a proportion, a ratio or a fraction as a whole number, by using 100 as the denominator. ... Population genetics is the study of the distribution of and change in allele frequencies under the influence of the four evolutionary forces: natural selection, genetic drift, mutation, and migration. ... Genetics (from the Greek genno γεννώ= give birth) is the science of genes, heredity, and the variation of organisms. ... In biology, a species is the basic unit of biodiversity. ... The gene pool of a species or a population is the complete set of unique alleles that would be found by inspecting the genetic material of every living member of that species or population. ...

Given a) a particular chromosome locus, b) a gene occupying that locus, c) a population of 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) and finally d) a variant or allele of the gene, then the allele frequency of that allele is the fraction or percentage of loci that the allele occupies within the population. Figure 1: Chromosome. ... The word locus (plural loci) is Latin for place: In biology and evolutionary computation, a locus is the position of a gene (or other significant sequence) on a chromosome. ... This stylistic schematic diagram shows a gene in relation to the double helix structure of DNA and to a chromosome (right). ... A somatic cell is generally taken to mean any cell forming the body of an organism: the word somatic is derived from the Greek word sōma, meaning body. Somatic cells, by definition, are not germline cells and cannot divide or differentiate to produce a new generation of offspring under... Diploid (meaning double in Greek) cells have two copies (homologs) of each chromosome (both sex- and non-sex determining chromosomes), usually one from the mother and one from the father. ... An allele is any one of a number of viable DNA codings of the same gene (sometimes the term refers to a non-gene sequence) occupying a given locus (position) on a chromosome. ...

To take an 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 variants of the gene, of which there may be one or many.

Note that for diploid genes, however, the proportion of individuals that carry this allele may be up to two in five. 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). So all together 36% of diploid individuals would be expected to carry an allele that has a frequency of 20%. However, alleles distribute randomly only in the absence of selection and under other assumptions. When these conditions apply, a population is said to be in Hardy-Weinberg equilibrium. In ordinary language, the word random is used to express apparent lack of purpose or cause. ... In mathematics, the binomial theorem is an important formula giving the expansion of powers of sums. ... Heterozygote cells are diploid or polyploid and have different alleles at a locus (position) on homologous chromosomes. ... Homozygote cells are diploid or polyploid and have the same alleles at a locus (position) on homologous chromosomes. ... Selection is hierachically classified into natural and artificial selection. ... Hardy–Weinberg principle for two alleles: the horizontal axis shows the two allele frequencies p and q, the vertical axis shows the genotype frequencies and the three possible genotypes are represented by the different glyphs The Hardy–Weinberg principle (HWP) (also Hardy–Weinberg equilibrium (HWE), or Hardy–Weinberg law) states...

The frequencies of all the alleles of a given gene often are graphed together as an allele frequency distribution histogram. 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. In statistics, a frequency distribution is a list of the values that a variable takes in a sample. ... In statistics, a histogram is a graphical display of tabulated frequencies. ... A speculative phylogenetic tree of all living things, based on rRNA gene data, showing the separation of the three domains, bacteria, archaea, and eukaryotes. ... Genetic drift is a contributing factor in biological evolution, in which traits which do not affect reproductive fitness change in a population over time. ... In biology, mutations are changes to the genetic material (usually DNA or RNA). ... This article is about non-human migration. ...

Calculation of Allele Frequencies

If f(A / A), f(A / a), and f(a / a) are the frequencies of the three genotypes at a locuswith two alleles, then the frequency p of the A allele and the frequency q of the a allele are obtained by counting alleles. Because each homozygote A/A consists only of A alleles, and because half of the alleles of each heterozygote A/a are A alleles, the total frequency p of A alleles in the population is calculated as

p=f(A/A)+ .5f(A/a)= frequency of A

Similarly, the frequancy q of the a allele is given by

p=f(a/a)+ .5f(A/a)= frequency of a

Therefore

p + q = f(A/A) + f(a/a) + f(A/a)= 1.00

and

q= 1- p

If there are more than two different allelic forms, the frequency for each allele is simply the frequency of its homozygote plus half the sum of the frequencies for all the heterozygotes in which it appears.

Compare genotype frequency. In population genetics, the genotype frequency is the frequency (or proportion i. ...

The Effect of Mutation on Allele Frequency

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 preceding meiosis). If p_t is the frequency of the A allele in generation t, if q_t=1-p_t 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 example), then the change in allele frequency in one generation is In genetics, mutation rates are the speed at which mutations in an organism or a species take place. ...

Δp=p_t - p_(t-1) = (p_(t-1) - ú(p_(t-1)) - p_(t-1) = -úp_(t-1)

where p_t-1 is the frequency of the preceding generation. 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 proportion p of all the genes that are still available to mutate. Thus Δp gets smaller as the frequency of p itself decreases, because there are fewer and fewer A alleles to mutate into a alleles. We can make an approximation that, after n generations of mutation,

p_n=p_oe^-nú

where e is the base of the natural logarithm.

Example

If there are ten individuals in a population and at a given locus there are two possible alleles, A and a, then if the genotypes of the individuals are: The genotype is the specific genetic makeup (the specific genome) of an individual, usually in the form of DNA. It codes for the phenotype of that individual. ...

AA, Aa, AA, aa, Aa, AA, AA, Aa, Aa, and AA

then the allele frequencies of allele A and allele a are:

p_A = (2+1+2+0+1+2+2+1+1+2)/20 = 0.7

p_a = (0+1+0+2+1+0+0+1+1+0)/20 = 0.3