In this article we will discuss about the co-dominance and incomplete dominance.
The phenotypic ratios obtained by Mendel in garden peas demonstrate that one gene controls one character; of the two alleles of a gene, one allele is completely dominant over the other. Due to this the heterozygote has a phenotype identical to the homozygous parent.
Soon after Mendel’s work was rediscovered, instances came to light where a gene was not producing an individual effect. On the contrary, genes were interacting with each other to produce novel phenotypes which did not exhibit dominance relationships observed in Mendel’s experiments.
In one of the first cases reported by Kolreuter, the heterozygote showed a phenotype intermediate between the parental phenotypes. This was termed incomplete dominance or intermediate inheritance. In co-dominance the heterozygote expresses both the parental phenotypes equally. Sometimes a gene masks the expression of another gene at a different locus. This is known as epistasis.
On still other occasions a gene does not completely mask another gene as in epistasis, but, in some way modifies the effect of the second gene. Known as modifying genes, such genes either enhance or suppress the expression of a different gene. Interaction between genes enables some genes to act together to produce an effect that neither gene can produce separately.
Such genes are said to be complementary. There are genes that copy other genes, so to say, to produce a similar effect. Thus independent genes that produce the same effect are given the name duplicate genes. Lastly, but most serious are genes causing death. They are known as lethal genes.
Co-Dominance:
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 F1 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 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 F1 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 recognised 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 F1 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, co-dominant genes segregate in the ratio 1:2:1.
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 N antigenic specificities expressed equally in the heterozygote.
Incomplete Dominance:
A monohybrid cross between a red-flowered snapdragon (Antirrhinum majus) and a white flowered variety does not produce red or white flowered plants in F1 as expected from mendelism. Instead the flowers are pink, i.e. intermediate between the two parents. This is because neither red flower colour nor white is dominant, but each allele has its influence in color development and the hybrid appears pink.
If the F1 pink flowers are self-pollinated, the F2 progeny shows red, pink and white flowered plants in the proportion 1: 2: 1. It may be recalled that this is the same genotypic ratio that Mendel obtained in garden peas. The difference is that in the present case the heterozygous progeny is distinct in appearance from the homozygotes.
The name intermediate inheritance is also given to crosses where F1 hybrids show incomplete or partial dominance with no phenotypic resemblance to either parent. This type of inheritance has been found in crosses involving many other plants.