Codominance.

Definition.

Different alleles of a gene that are both expressed in a heterozygous condition are called codominant. Explanation. The phenotype of heterozygote is distinct in quality from those of the two homozygotes. It is not an intermediate quantitative expression like incomplete dominance. Each allele of the gene pair is associated with a different

substance, e.g.,

Codominance occurs when both the alleles express independently in heterozygote; (A|A2) and form their respective products X and Y. The codominant heterozygote would have both substances at the same time Example. (MN BLOOD GROUP.) Human blood groups can be of many types, e.g. ABO, MN, MNSs, Rh etc. Discovery.

Landsteiner and Levine discovered MN blood types in man on the basis of specific antigens present on RBC. These RBC antigens induce production of their specific antibodies.

Phenotypes.

There are three general phenotypes;

M,N and MN.

1. M

phenotype has antigen M which is produced by gene L M .

2. N

phenotype has antigen N that is produced by its allele L N .

3. MN

phenotype has both M and N antigens, simultaneously produced by their alleles L M and L N If a man of M blood group marries a woman of N blood group, all their children will have MN blood group.