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In population genetics, a fixed allele is an allele that is the only variant that exists for that gene in a population. A fixed allele is homozygous for all members of the population. [1] The process by which alleles become fixed is called fixation. For this hypothetical species, the population in the topmost frame exhibits no fixed allele for ...
In the absence of mutation or heterozygote advantage, any allele must eventually either be lost completely from the population, or fixed, i.e. permanently established at 100% frequency in the population. [2] Whether a gene will ultimately be lost or fixed is dependent on selection coefficients and chance fluctuations in allelic proportions. [3]
Population genetics is a subfield of genetics that deals with genetic differences within and among populations, and is a part of evolutionary biology.Studies in this branch of biology examine such phenomena as adaptation, speciation, and population structure.
If ¯ is the average frequency of an allele in the total population, is the variance in the frequency of the allele among different subpopulations, weighted by the sizes of the subpopulations, and is the variance of the allelic state in the total population, F ST is defined as [2]
Once an allele becomes fixed, genetic drift comes to a halt, and the allele frequency cannot change unless a new allele is introduced in the population via mutation or gene flow. Thus even while genetic drift is a random, directionless process, it acts to eliminate genetic variation over time.
The neutral theory assumes that most mutations that are not deleterious are neutral rather than beneficial. Because only a fraction of gametes are sampled in each generation of a species, the neutral theory suggests that a mutant allele can arise within a population and reach fixation by chance, rather than by selective advantage. [1]
Punnett square for three-allele case (left) and four-allele case (right). White areas are homozygotes. Colored areas are heterozygotes. Consider an extra allele frequency, r. The two-allele case is the binomial expansion of (p + q) 2, and thus the three-allele case is the trinomial expansion of (p + q + r) 2.
The allele that is linked to the adaptation will increase in frequency, in some cases until it becomes fixed in the population. The other allele, which is linked to the non-advantageous version, will decrease in frequency, in some cases until extinction. [5] [6] Overall, hitchhiking reduces the amount of genetic variation.