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Incomplete dominance: a deviation from Mendel's laws
Mendel’s laws are surprisingly simple and fundamental. So it shouldn’t come as a surprise that, with the rediscovery of the Austrian monk's work in the early twentieth century, scientists wanted to test how universal these laws actually were. Quickly, it became obvious that there were numerous situations in which his laws no longer seemed to apply.
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Some of these deviations are caused by variations in the dominance relations between alleles of the same gene. One of these variations is called incomplete dominance (or semidominance). With complete dominance, which was the case in Mendel’s experiments, the heterozygote Aa has the same phenotype as the homozygote AA. In that case, A is called dominant. When the alleles are subject to incomplete dominance, the heterozygote Aa has an intermediary phenotype in relation to the homozygotes AA and aa. Incomplete dominance is thus defined as the inability of an allele to fully express the homozygote phenotype in heterozygote individuals, which results in an intermediary phenotype.
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This intermediary expression of a certain inheritable trait can lean towards one of the two possible homozygote phenotypes as a result of the relative proportion that each of the two alleles contributes to the eventual phenotype. Say, for example, that the relative contribution of alleles A and B is 60:40. In this case, the heterozygote (AB) phenotype will look more like the AA homozygote than like the BB individual. If both alleles provide an equal contribution (50:50), than the heterozygote phenotype will be an exact intermediary between the two homozygote phenotypes. In each case, the result of incomplete dominance is that a monohybrid cross-fertilization between two homozygotes will give rise to a heterozygote F1 generation with a phenotype that differs from both parents.
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An example: Genetics of Black and Blue Chickens
An example of this incomplete dominance is found in the feather color of black and blue chickens. The black color of the feathers in black individuals is caused by melanin (white feathered animals do not produce melanin). The monohybrid cross-fertilization between a black homozygote (BB) individual and a white (bb) one, leads to an F1 generation exclusively consisting of heterozygote ‘blue’ (Bb) chickens.
F0: BB x bb
F1: 100% Bb
When these F1 individuals are crossbred, the resulting F2 generation will be made up by one quarter of black chickens (BB), a half of the chickens will be ‘blue’ (Bb) and the remaining quarter will have a white feather color (bb).
F1: Bb x Bb
F2: 25% BB, 50% bB, 25% bb
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A while ago, this incomplete dominance was considered to be evidence for the 'mixing theory' which states that the descendants of two different homozygotes would have a phenotype that is a 'mix' of both homozygote phenotypes. However, the segregation in the F2 generation proves that this is not the case because, if the 'mixing theory' were correct, all F2 individuals should be 'blue'.
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Crawford, Roy (1990). Poultry Breeding and Genetics. Elsevier
Hutt, Frederick Bruce (1949). Genetics of the Fowl. New York: McGraw-Hill.