This condition is called lack of dominance and it can happen in two ways: incomplete dominance or codominance.
In incomplete dominance the heterozygous presents an intermediate phenotype between the two types of homozygous, as in sickle cell anemia in which the heterozygous produces some sick red blood cells and some normal red blood cells. Codominance occurs, for example, in the genetic determination of the MN blood group system, in which the heterozygous has a phenotype totally different from the homozygous, not being an intermediate form.
