What is codominance in genetics?

In 1900 Bateson and his colleagues studied inheritance of comb shape in fowls. There are four types of combs in fowls: rose, pea, walnut and single. Bateson first performed a cross between rose and single. The Fx hens all had a rose comb, and on inbreeding gave rise to an F2 progeny of rose and single in the ratio 3 : 1. The cross indicates that rose and single comb are controlled

comb-types-in-fowl
comb-types-in-fowl

by a single gene and that rose is dominant over single. In the second cross when chickens with pea comb were mated with single comb, the Fx progeny had pea comb, and F2 had pea and single in the proportion 3 : 1. Obviously, the gene for pea comb is dominant over single. This raises an interesting question—are the genes for rose and pea comb same or different? Bateson then crossed rose and pea. Surprisingly, the F1 birds had an altogether different comb of the

walnut type! An F2 progeny raised by inbreeding the walnut type consisted of four types of chickens—walnut, rose, pea and single (Fig. 2.1) in the ratio 9 : 3 : 3 : 1. As this ratio is typical for dihybrid inheritance it became clear that rose and pea combs were controlled by two pairs of genes.

The appearance of walnut comb in F1 of cross between rose and pea shows that both of the independent dominant genes P and R are jointly responsible for the walnut comb. When present together in the zygote, P and R genes interact to produce the walnut comb. When present alone, they produce rose or pea comb. The recessive alleles of rose and pea combs produce the fourth type of chicken with the single comb.

Another example where the heterozygote can be recognized distinctly is offered by the “roan” (reddish grey) coat colour of short horn cattle. When homozygous red-haired cattle are crossed with homozygous white-haired type, the F^ has reddish grey hair and is designated “roan”. It must be noted that there is no mixture of red and grey pigments in a roan. But some hair are all red, others all white, so that the final effect is a reddish grey coat colour. The cross therefore demonstrates a difference from intermediate inheritance. In F2, codominant genes segregate in the ratio 1:2: 1.

P: Red    x     White

r1r1        r2r2

F1:      “Roan” (reddish grey)    x       “Roan”

F2 :    1 Red :     2 “Roan” :     1 White

The ABO blood group system in man is controlled by multiple alleles of a gene I, each allele producing a different antigen. Likewise there are four blood group phenotypes in man designated A, B, AB and O. Individuals with blood group O have no antigen, whereas those with AB represent heterozygotes in which both A and B antigens are present. The AB heterozygote has both dominant alleles IA and IB equally expressed to produce distinct A and B antigens. Similarly, the rare blood group MN also shows both M and A7 antigenic specificities expressed equally in the heterozygote.

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