Incomplete dominance and codominance are two patterns of inheritance that occur when alleles at a single locus do not exhibit complete dominance as described by Mendel’s Law of Dominance. In these cases, the heterozygous genotype expresses a phenotype that is intermediate between the phenotypes of the two homozygous genotypes.
Incomplete dominance occurs when the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes. For example, in a cross between a red flower (RR) and a white flower (WW) with incomplete dominance, the F1 generation will have pink flowers (RW). The pink phenotype is intermediate between the dominant red and recessive white phenotypes, and results from the production of a protein or RNA molecule that has a reduced level of activity, leading to a partial expression of the dominant trait.
Codominance, on the other hand, occurs when both alleles of a heterozygous genotype are fully expressed in the phenotype. In this case, the heterozygous phenotype shows both traits simultaneously. An example of codominance is the human blood group AB, where both the A and B alleles are expressed on the surface of red blood cells, resulting in a phenotype with both A and B antigens.
In some cases, it can be difficult to distinguish between incomplete dominance and codominance. One way to tell them apart is by observing the F2 generation in a cross between two heterozygotes. In incomplete dominance, the F2 generation will have a 1:2:1 ratio of phenotypes, with one homozygous dominant, two heterozygotes, and one homozygous recessive. In codominance, the F2 generation will have a 1:2:1 ratio of phenotypes, but all three phenotypes will be present and expressed equally.
Both incomplete dominance and codominance can have important implications in genetics, including in the expression of physical traits, the inheritance of genetic diseases, and the breeding of plants and animals for desirable traits. Understanding these patterns of inheritance is crucial for predicting and interpreting the outcomes of genetic crosses and for advancing the field of genetics.