||In some genetic
systems there are more than one pair of alleles. Also the genetic
system doe not always reflect a pure dominance/ recessive relationship.
A case in point would be the ABO blood groups seen in Humans.
In humans there are three alleles that determine blood types. They are A, B and O alleles. In this system, A is dominant to O and B is dominant to O. But, A is not dominant to B or vice versa. An individual with Blood type A can be AA or AO, A person with type B blood can be either BB or BO. But when A and B alleles are seen together a third phenotype is seen. That being AB type blood. This is incomplete dominance. There is also a fourth phenotype possible, that being type O blood which can only occur if the genotype is homozygous O.