Fixed Allele
A fixed allele refers to a form of a gene (allele) carried by all members of a population at all loci for that particular gene. Accordingly, with regard to a fixed allele, all individuals within a population will be homozygous for that allele and no other alleles of that particular gene will exist within the population.
Because alleles refer to differing forms of a particular gene that controls a specific character or trait, when allele fixation takes place within a population there are no alternative forms of the gene available to subsequent offspring. Aside from mutational changes, once an allele is fixed within a population, the normal laws of Mendelian inheritance dictate that all future generations will carry only that allele.
Organisms in the diploid state (e.g., genetically normal humans) normally carry two alleles, one at a specific place (gene locus) on each of two homologous chromosomes. Heterozygotes carry identical copies of alleles at both loci and heterozygous organisms carry differing alleles. Because allele fixation means that all individuals with a population carry a specific form of a gene -- and only that form of the gene -- the entire population if homozygous for that gene.
Changes in gene frequency are a natural result of real population sizes. To maintain the frequencies of alleles predicted by the Hardy-Weinberg equation, for example, there is an underlying assumption that the population under study is of infinite size. As populations sizes decrease the importance of genetic drift and chance play an increasingly important role in allele frequencies within the population. Even assuming that genetic drift is random (i.e., due to random factors a given allele frequency alternatively increases or decreases from one generation to the next) eventually such a drift -- given sufficient time -- would either eliminate an allele from a population or cause the allele to be fixed within the population. When the allele is either eliminated or fixed, no further change is possible until a mutation produces a new allele.
The smaller the population size, the larger are the fluctuations in gene frequencies between generations. Accordingly, small populations tend to show a greater loss of heterozygous organisms and have a greater chance of undergoing allele fixation.
The greater the mutation rate in an allele, the less likely it is to undergo fixation. As genetic drift and random chance eliminate or fix alleles within a population, the processes of mutation supply new alleles that are subject to the forces of selection.
In addition to fixed alleles that arise by drift, natural selection pressures may act to eliminate or fix and allele within a given population.
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