Explain how many phenotypes are there for a trait that has two alleles that are governed by incomplete dominance

The answer is 3 phenotypes. These are the homozygous dominant, homozygous recessive and heterozygous. In incomplete dominance, the dominant traits do not fully mask the recessive gene (note this is different from co-dominance). Example of incomplete dominance in humans is the alleles for straight and curly hair. Heterozygous for this trait is wavy hair due to incomplete dominance.