What is incomplete dominance?

Sometimes, the combined expression of two different alleles in the heterozygous condition produces a blending of the individual expressions of the two alleles called incomplete dominance.
In snapdragons, for example, the heterozygous condition consisting of one allele for red flowers (R) and one allele for white flowers (r) is pink (Rr). Sometimes, both alleles are written with the same uppercase or lowercase letter but with a prime or superscript number or letter to differentiate the two. As an example, R and R' might represent red and white alleles for snapdragons. As another example, H and H1 might represent straight and curly hair alleles in humans. The HH1 phenotype is expresses as an intermediate trait, wavy hair. In still other cases, there may be no apparent rationale, except perhaps historical, for the notation used to indicate different alleles.