Intermediate Inheritance (Incomplete Dominance)
From Mendel's experiments it had been established that when two alleles are brought together from two different pure breeding parents, one of them completely dominates over the other manifesting itself in the hybrid. Researches by many investigators revealed that in a number of living organisms complete dominance was absent, the hybrid exhibited an intermediate character as both the genes of the allelomorphic pair showed partial expression.
Thus in incomplete dominance or partial dominance or intermediate inheritance or blending inheritance the F1 hybrid does not resemble either of the parents. A very good example for this is the 4 `O' clock plant Mirabilis jalapa studied by Correns in 1906. A similar condition is seen in Antirrhinum majus.
In Mirabilis jalapa, there are two distinctive types of flower colours namely the red and the white. Both the types are true breeding. When a pure-red flowered (r1r1) variety is crossed with a pure white flowered (r2r2) one, the F1 hybrids produce pink flower, a character which is intermediate between red and white coloured flowers of the parental generation. This is because neither red flower colour nor white is completely dominant over the other. When F1 hybrids were selfed red, pink and white flowered varieties were obtained respectively in the ratio of 1:2:1. This is the phenotypic ratio. Here the genotypic ratio is also 1:2:1, producing one homozygous red, two heterozygous pink and one homozygous white. The red and the white varieties breed true on self fertilisation of the F2 individuals but the pink varieties on selfing once again produce a phenotypic ratio of 1:2:1 proving the law of purity of gametes.
Since neither of the parents is completely dominant over the other, the symbol for Red parent is r1r1 and the white parent r2r2, and so naturally the genotype of the hybrid is r1r2.
It has been observed in the given example that there is blending of phenotypes - not genotypes and the alleles of the genes are discrete or particulate. They appear blended in F1 but have separated out in F2 generation.
Incomplete dominance is also called blending inheritance because both the characters of the parental plants are mixed to give an intermediate character which is different from that of the parents. But only the characters are mixed with each other and not the alleles. In Mirabilis the r1r1 always produces red coloured flowers and r2r2 produces white coloured flowers, when they are combined, the intermediate colour namely pink is produced. Because of this it is described as blending inheritance.
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