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| Incomplete Dominance |
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| It is a type of intragenic (or interallellic) interaction where both the alleles of a given trait express as a blend (mixture) as against a normal Mendelian pattern where one allele is dominant over the other. As a resulting of this blending, an intermediate character is expressed. This situation occurs due to the fact that the dominant gene is not in a position to completely suppress the expression of recessive gene. With the result, the heterozygous offspring will be phenotypically and genotypically different from either of the homozygous parent. Following are the two familiar examples of incomplete dominance. |
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| In the plant Mirabilis jalapa, commonly called as four'o clock plant, the inheritance of flower colour is an example for incomplete dominance. |
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| The plant produces two types of flowers red coloured and white coloured. This condition is an example for a pair of contrasting characters. When a plant which is homozygous for red flowers (AA) is crossed with a plant which is homozygous for white flowers (aa), the plants of the F1 generation produce pink flowers which is a blend of red and white condition. This result clearly indicates that neither red flowered condition nor white flowered condition is dominant. However, when two hybrid plants with pink flowers (Aa) are crossed, the F2 generation plants show red flowered, pink flowered and white flowered condition in the ratio 1:2:1. This ratio is very much in accordance with the law of segregation. |
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| fig. 8.12 Flower Colour in Four O Clock Plant |
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| This example very clearly indicates |
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 The phenomenon of incomplete dominance |
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 That the genes responsible for red and white flowers do not actually mix, since both the pure characters reappear in the F2 generation |
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 That there is no specific gene responsible for producing pink flowers |
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 That the homozygous white flowered plants have genes aa which is unable to produce the colouring pigment |
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 That the heterozygous pink flowered plants have genes Aa and hence can produce only half the amount of colouring pigment that is normally produced in a red flowered plant (AA) |
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| The Andalusian fowl has three varieties splashed white, black and blue. A cross between a pure splashed white male fowl and a pure black female fowl yields, hybrid fowls with blue colour, in the F1 generation. When the blue hybrid fowls are crossed, with each other, the F2 generation shows fowls with splashed white, blue and black colour in the ratio 1:2:1. |
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| The condition of sickle cell anaemia in human beings is also an example of incomplete dominance. |
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