[IB Biology Genetics Notes]

Codominant Traits

A number of human traits are the result of 2 types of alleles that are equally dominant. Such traits are said to be codominant for that trait. When an individual is heterozygous for such traits, the resulting phenotype or expression of these two traits is a blending, because both traits are expressed equally.

The alleles for curly hair and straight hair are examples of alleles for a trait that are codominant. Individuals with curly hair are homozygous for curly hair alleles. Individuals with straight hair are homozygous for straight hair alleles. Individuals who are heterozygous, with one of each allele have wavy hair, which is a blend of the expressions of the curly and straight hair alleles.

Sample Codominance Problems

In horses, chestnut and white coat colors are codominant. Heterozygous horses have a blend of both colors, which is a golden tan color. Such heterozygous horses are known as palominos (like Mr. Ed).