Incomplete dominance, also known as partial dominance, happens when two alleles produce protein, but one does not function the way that it should. While in some cases this situation doesn't affect a person, other times it is the leading cause of birth defects or infant death.
This kind of gene expression can be harder to see in humans than in other organisms. A common example that is used to show incomplete dominance is the color in snapdragon leaves. When a red and a white flower's genes are put together, there is one gene for red flowers and one for white. The expression of incomplete dominance is seen when the flowers that bloom turn out pink.