The presence of certain alleles on the ABO gene determines blood type. An allele is a piece of genetic information found on a specific chromosome, as defined by Arizona State University. There are six possible genetic combinations for blood type in humans.
The three alleles responsible for a person's blood type are A, B and O. A developing baby gets one of these alleles from his mother and one from his father, so the six possible combinations are AA, BB, AB, AO, BO and OO. O is recessive, so the only way to have type O blood is to inherit O alleles from both parents, reports Arizona State University.
If an O allele pairs with an A or B allele, the baby has the blood type corresponding to the dominant allele. For example, someone with A and O alleles has type A blood, according to Arizona State University. The A and B alleles are equally dominant, so someone with an A and a B has type AB blood. It is also possible for two of the same alleles to pair together, resulting in a baby with an AA or BB genotype.
The presence or absence of Rh antibodies determines if someone has a positive or negative blood type, reports WebMD. A person with Rh antibodies in his blood is Rh-positive, while someone without the antibodies is considered Rh-negative. Therefore, there are a total of eight blood types: A-negative, A-positive, B-negative, B-positive, AB-negative, AB-positive, O-positive and O-negative, states TeensHealth.