In incomplete dominance (sometimes called partial dominance), a heterozygous genotype creates an intermediate phenotype. In this case, both the dominant and recessive alleles are expressed, creating a blended or combined phenotype. A cross of two intermediate phenotypes can result in the reappearance of either the parent phenotypes or the blended phenotypes. An example of incomplete dominance in humans is mordan, a trait that is exhibited when eye color alleles from the maternal and paternal chromosomes are blended. This usually occurs when one parent has green eyes and the other parent has brown eyes–the child will have dark blue eyes.

In co-dominance, neither phenotype is dominant. Instead, the individual expresses BOTH phenotypes. The most important example is in Landsteiner blood types. The gene for blood types has three alleles: A, B, and i. i causes O type and is recessive to both A and B. When a person has both A and B, they have type AB blood.