Resources

Resource 1: Worksheet for Activity 1

Complete dominance occurs where one completely dominant allele overrides the effect of a recessive second allele. The result is that there are only ever two phenotypes present in the offspring. However, codominance occurs where two alleles are expressed in the same phenotype. For example, the carnation plant can have red, white or pink flowers. This is because neither the red nor the white alleles are fully dominant. This means that if a red plant is bred with a white plant, the resulting F1 offspring would have pink flowers. Wherever you find a third phenotype present, this means that there is codominance present.

A further example of codominance can be found in cats. If a black cat and ginger cat mate, the kitten would have both black fur and ginger fur. Codominance can also occur in blood types, Type AB is codominant because both the antigen A and antigen B show up in the genotype.