Incomplete Dominance: Correns discovered incomplete dominance. It is the phenomenon where dominant alleles do not completely express itself. This phenomenon was first studied in flower colour of Mirabilis jalapa or Four O’clock plant. In this plant, red flowers are incompletely dominant over white flowers, the heterozygotes being pink.
Co-Dominance: When the heterozygote exhibits a mixture of phenotypic characters of both homozygotes.
In shorthorn cattle, alleles for red and white coat colour occur. Crosses between red (r1r1) and white (r1r2) coat. A close examination of hairs of roan animals reveals that the coat is made up of a mixture of red hairs and white hairs. Here co-dominance occurs, rather than intermediate dominance.
Incomplete Dominance vs Co-Dominance
1. Effect of one of the two alleles is more conspicuous.
2. It produces a fine mixture of the expression of two alleles.
3. The effect in hybrid is intermediate of the two alleles.
4. The expressed phenomenon is new. It has no allele of its own.
5. The incompletely dominant allele has quantitative effect.
1. Effect of both the alleles are equally conspicuous.
2. There is no mixing of the effect of the two alleles.
3. Both the alleles produce their effect independently. Ex IA and IB, HbS and HbA .
4. The expressed phenotype is combination of two phenotypes and their alleles.
5. A quantitative effect is absent.