Codominant alleles Codominant alleles occur when rather than expressing an intermediate phenotype, the heterozygotes express both homozygous phenotypes. An example is in human ABO blood types, the heterozygote AB type manufactures antibodies to both A and B types. Codominant alleles are both expressed. Heterozygotes for codominant alleles fully express both alleles. Blood type AB individuals produce both A and B antigens.
Incomplete dominance is a condition when neither allele is dominant over the other.
The condition is recognized by the heterozygotes expressing an intermediate phenotype relative to the parental phenotypes. If a red flowered plant is crossed with a white flowered one, the progeny will all be pink. Image from W.H. Freeman and Sinauer Associates, used by permission. An explanation is available by clicking here . Flower color in snapdragons is an example of this pattern.