Codominance

What codominance means

In codominance, neither allele is dominant over the other, and both are expressed fully and simultaneously in the heterozygote, the two traits appear side by side or together, not blended into an intermediate. This is the key difference from incomplete dominance, where the heterozygote shows a single blended phenotype.

Worked problem 2 (medium): roan coat colour in cattle

Coat colour in some cattle breeds is controlled by alleles C^R (red hair) and C^W (white hair). A homozygous red animal (C^R C^R) crossed with a homozygous white animal (C^W C^W) produces roan offspring (C^R C^W), an animal with red hairs and white hairs both present in its coat, rather than pink.

Worked example: AB blood group

The ABO blood group system also shows codominance. The alleles I^A and I^B are codominant with each other (though both are dominant over allele i).

A person with genotype I^A I^B expresses both A and B antigens on their red blood cells at the same time, giving blood group AB, not a blend of A and B.

Codominance vs incomplete dominance

Worked problem 1 (easy): pure-breeding red x pure-breeding white

A homozygous red bull (C^R C^R) is crossed with a homozygous white cow (C^W C^W). The bull's gametes all carry C^R and the cow's gametes all carry C^W, so every calf is heterozygous C^R C^W.

Worked problem 3 (SPM level): roan x white

A roan bull (C^R C^W) is crossed with a white cow (C^W C^W). The bull produces C^R and C^W gametes, while the cow produces only C^W gametes.

The offspring are 1 roan (C^R C^W) : 1 white (C^W C^W), a phenotype ratio of 1 roan : 1 white. No calf is fully red, because the white parent cannot contribute a C^R allele.

Practice problems

Set out each cross yourself, then check your working against the answers below.

• 1. Cross a homozygous red bull (C^R C^R) with a roan cow (C^R C^W). State the offspring phenotypes and the ratio.
• 2. Two roan cattle are crossed. What fraction of the calves are expected to be white?
• 3. A person of blood group AB has children with a person of blood group O. State the children's blood groups and explain whether codominance is seen in any child.

Answers

Common mistakes

Source:SRC-DSKP-EN

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