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Codominance

In codominance two alleles are fully expressed at the same time in a heterozygote. Codominant alleles are written as capitals with subscripts or superscripts. A good example of codominance is that of the roan coat as seen in some cattle and horses.

This animation shows a cross between a white homozygous horse and a red homozygous horse, which results in a heterozygous offspring with a roan coat. This roan coat consists of a mixture of red and white hairs where each individual hair is either all white or all red.

Since crosses involving traits that show incomplete dominance or codominance generate the same genotypic and phenotypic ratios in the progeny, it is often difficult to tell these two types of inheritance apart.