An example of a Punnet square that demonstrates codominance is made up of offspring that have blood type AB. A simple cross involves the parents being homozygous for dominant A (IA) or dominant B (IB). The genotypes of these parents are (IA, IA) and (IB, IB), where the resulting offspring all have a genotype of (IA, IB) making the phenotype of blood type AB.
In the above example, all the offspring have the genotype (IA, IB) and the phenotype of blood type AB. Other Punnet squares also yield offspring with genotypes (IA, IB) as long as both parents lack the recessive allele (i) in their genotypes.
Human blood genotypes comprise three alleles that include dominant A (IA), dominant B (IB) and a recessive allele (i). Blood type is determined by the combinations of these alleles. A human with blood type A has a genotype of (IA,IA) or (IA,i), while a human with blood type B has a genotype of (IB, IB) or (IB, i). Humans with blood type AB lack the recessive allele, and their genotype is (IA, IB). Blood type AB is inherited in a codominant pattern. This means that both alleles in the gene pair are expressed fully, so neither allele is more dominant than the other.