There are some alleles which are incompletely dominant. This happens in blood type, for example. There are three alleles: type A, type B, and type O. A and B are dominant over O. Thus, if your genotype is AO, or AA you are type A. If your genotype is BO or BB, you are type B. If your genotype is OO, you are type O. However, if your genotype is AB, you have neither type A nor type B blood. You have a different blood type, called AB.

Incomplete dominance is discussed at the end of Biology Module 8, and the example of the snapdragon flower is given.