The Human ABO markers: The A, B, and O alleles
Human blood type is determined by co-dominant alleles. An allele is one of several different forms of genetic information that is present in our DNA at a specific location on a specific chromosome. There are three different alleles for human blood type, known as IA, IB, and i. For simplicity, we can call these alleles A (for IA), B (for IB), and O (for i).
Each of us has two ABO blood type alleles, because we each inherit one blood type allele from our biological mother and one from our biological father. A description of the pair of alleles in our DNA is called the genotype. Since there are three different alleles, there are a total of six different genotypes at the human ABO genetic locus. The different possible genotypes are AA, AO, BB, BO, AB, and OO.
How are blood types related to the six genotypes?
A blood test is used to determine whether the A and/or B characteristics are present in a blood sample. It is not possible to determine the exact genotype from a blood test result of either type A or type B. If someone has blood type A, they must have at least one copy of the A allele, but they could have two copies. Their genotype is either AA or AO. Similarly, someone who is blood type B could have a genotype of either BB or BO.
A blood test of either type AB or type O is more informative. Someone with blood type AB must have both the A and B alleles. The genotype must be AB. Someone with blood type O has neither the A nor the B allele. The genotype must be OO.
How are ABO alleles inherited by our children?
Each biological parent donates one of their two ABO alleles to their child. A mother who is blood type O can only pass an O allele to her son or daughter. A father who is blood type AB could pass either an A or a B allele to his son or daughter. This couple could have children of either blood type A (O from mother and A from father) or blood type B (O from mother and B from father).
Since there are 4 different maternal blood types and 4 different paternal blood types possible, there are 16 differnt combinations to consider when predicting the blood type of children. In the tables below, all 16 possible combinations are shown. If you know the blood type of the mother and father, the possible blood types for their children can be found.
What about the Rh factor? Can a father of blood type A+ have a child who is blood type A-?
The Rh factor genetic information is also inherited from our parents, but it is inherited independently of the ABO blood type alleles. There are 2 different alleles for the Rh factor known as Rh+ and Rh-. Someone who is "Rh positive" or "Rh+" has at least one Rh+ allele, but could have two. Their genotype could be either Rh+/Rh+ or Rh+/Rh-. Someone who Rh- has a genotype of Rh-/Rh-.
Just like the ABO alleles, each biological parent donates one of their two Rh alleles to their child. A mother who is Rh- can only pass an Rh- allele to her son or daughter. A father who is Rh+ could pass either an Rh+ or Rh- allele to his son or daughter. This couple could have Rh+ children (Rh- from mother and Rh+ from father) or Rh- children (Rh- from mother and Rh- from father).
Answering the Question from the Mother in Alberta, Canada
The mother in question is blood type A+. Her genotype at the ABO location is either AA or AO. Her Rh genotype is either Rh+/Rh+ or Rh+/Rh-. The information that the maternal grandmother is also blood type A+ and a brother is blood type O tells us that the maternal grandmother of the child has genotype AO, since she is type A but donated an O allele to one of her children.
The mother wants to know the potential blood types of the father of her son. The son is blood type A+. Unfortunately for this particular case, the mother cannot distinguish between any potential fathers from blood type alone. Note from the table that this mother could have created a child with type A blood with a father of any of the four possible blood types, typeA, type AB, type B, or type O. Likewise, the father of the child could be either Rh+ or Rh-.
It should be apparent from this discussion that blood type is not a very good test for paternity. In some cases, unambigous information can be obtained, i.e. a type AB male cannot father a type O child. However in most cases, the results are uncertain.
If determining the paternity of a child is important, there are very sensitive DNA test currently available that can establish paternity to a certainty in excess of 99.99%, or exclude someone as the biological father with absolute certainty. Elsewhere in the Biology Project is an excercise to follow the inheritance of DNA markers in a paternity study.
