the complete loss of an allele is called fixation.
what is the complete loss of an allele called?
fixation
bullet::::- genetic drift—genetic variation is determined by the joint action of natural selection and genetic drift (chance). in small populations, selection is less effective, and the relative importance of genetic drift is higher because deleterious alleles can become more frequent and 'fixed' in a population due to chance. the allele selected for by natural selection becomes fixed more quickly, resulting in the loss of the other allele at that locus (in the case of a two allele locus) and an overall loss of genetic diversity. alternatively, larger populations are affected less by genetic drift because drift is measured using the equation 1/2n, with "n" referring to population size; it takes longer for alleles to become fixed because "n" is higher. one example of large populations showing greater adaptive evolutionary ability is the red flour beetle. selection acting on the body color of the red flour beetle was found to be more consistent in large than in small populations; although the black allele was selected against, one of the small populations observed became homozygous for the deleterious black allele (this did not occur in the large populations). for any allele—deleterious, beneficial, or neutral—is more likely to be lost from a small population (gene pool) than a large one. this results in a reduction in the number of forms of alleles in a small population, especially in extreme cases such as monomorphism, where there is only one form of the allele. continued fixation of deleterious alleles in small populations is called muller's ratchet, and can lead to mutational meltdown.
small population size
bullet::::2. less commonly, the presence of a single functional allele gives a phenotype that is not normal but less severe than that of the non-functional homozygote. this occurs when the functional allele is not haplo-sufficient. the terms haplo-insufficiency and incomplete dominance are typically applied to these cases. the intermediate interaction occurs where the heterozygous genotype produces a phenotype intermediate between the two homozygotes. depending on which of the two homozygotes the heterozygote most resembles, one allele is said to show "incomplete dominance" over the other. for example, in humans the "hb" gene locus is responsible for the beta-chain protein (hbb) that is one of the two globin proteins that make up the blood pigment hemoglobin. many people are homozygous for an allele called "hb"; some persons carry an alternative allele called "hb", either as homozygotes or heterozygotes. the hemoglobin molecules of "hb"/"hb" homozygotes undergo a change in shape that distorts the morphology of the red blood cells, and causes a severe, life-threatening form of anemia called sickle-cell anemia. persons heterozygous "hb"/"hb" for this allele have a much less severe form of anemia called sickle-cell trait. because the disease phenotype of "hb"/"hb" heterozygotes is more similar to but not identical to the "hb"/"hb" homozygote, the "hb" allele is said to be "incompletely dominant" to the "hb" allele.
dominance (genetics)