Blood type is a trait with three different alleles. These are A, B, and O. When a sperm and egg unite at fertilization, one blood type allele is provided by each sex cell. The combination of the two alleles a person inherits determines which of the antigens will be present in their blood. Therefore, four possible blood types exist. These are A,B, AB, or O. The allele for A and B are codominant and both are dominant to O, which is recessive. Therefore, if a person inherits AA alleles, they will be homozygous type A. If they inherit AO, they will still have type A blood because A is dominant to O, but they will be heterozygous for their blood type. BB and BO are both type B individuals. AB alleles produce type AB blood--both alleles are expressed because they are codominant. However, the only way to get a type O individual, is two recessive O genes. Therefore, a type O individual is homozygous for blood type.