Co-dominance is a condition in which both alleles of a gene pair in heterozygous condition are fully expressed, with neither one being dominant or recessive to the other. Thus, in co-dominance, we get a blending of dominant and recessive traits resulting in different phenotype.

Example 1:

Blood group ‘AB’ in humans is an example of co-dominance.

i. Blood group character is controlled by gene I, that exists in three allelic forms I^A, I^B and I^O.

ii. In I^A and I^B , superscripts A and B stand for glycoproteins (sugar polymers) that are found projecting from the surface of RBCs.

iii. The allele I^A produces glycoprotein A, while I^B produces glycoprotein B, allele I^O does not produce any of them.

iv. The allele I^A is dominant over I^O. I^B is also dominant over I^O. Allele I^A and I^B are co-dominant and express themselves when present together. Such RBCs have both the types of glycoproteins and blood group will be AB.

Example 2:

Roan coat colour in Cattle.

i. There are two types, one with red coat (skin with red colour hair) and the other with white coat (with white hair).

ii. When red cattle (RR) is crossed with white cattle (WW), F₁ hybrids (RW) have roan colour. Roans have the mixture of red and white colour hair.

iii. Thus, both the traits are expressed equally. In F₂ generation (produced by interbreeding of roans), red (RR), roans (RW) and white (WW) are produced in the ratio 1:2:1.

iv. Thus, in co-dominance also, genotypic and phenotypic ratios are identical.