codominance

Meaning: Codominance is an inheritance pattern in which both alleles of a gene are fully expressed in a heterozygote, so both traits appear at once instead of one masking the other. Each allele makes its own distinct product. In ABO blood type, IA and IB encode different enzymes that add different sugars to red cell surface antigens, so an IAIB person has both A and B antigens (type AB). The i allele is recessive to both.

On the MCAT: Expect a cross or pedigree that asks for genotype or phenotype ratios. Two IAIB parents give 1 IAIA : 2 IAIB : 1 IBIB, a 1:2:1 phenotype ratio instead of the 3:1 seen with complete dominance. A type AB parent and a type O parent can have only type A or type B children. Transfusion questions use the fact that type AB red cells carry both antigens and the plasma has neither anti-A nor anti-B antibodies.

Common confusion: Codominance is not incomplete dominance. In incomplete dominance the heterozygote is a blend, such as pink flowers from red and white snapdragon parents. In codominance both parental traits show distinctly, such as red and white patches in roan cattle. Both patterns give a 1:2:1 phenotype ratio in a monohybrid cross of two heterozygotes, so decide by how the heterozygote looks.