Thanks to discovery's by Dr. Landsteiner, he made it possible to determine a person's blood group based on a characteristic of specific substances on the surface of red blood cells. The two most important classifications to describe blood groups in humans are A, B, O and the Rhesus factor +/- (Rh factor). Medically it is important to know your blood group as is the case with of blood transfusion, it is critical that a person gets the same or compatible blood type.
ABO and Rh blood systems for determination of paternity are not conclusive and at best can only be predictive. Blood grouping can be flawed due to mutations and give false conclusions and for these reasons we caution all readers to view this calculator only as a predictive tool.
In genetics, blood type gene has two alleles, each allele has genotype A, B or O. The A and B are dominant, and O is recessive. So allele A combined with allele O is type A. Similarly, BO is type B, AA is type A, BB is type B, OO is type O, and AB is type AB.
If both parents have type A blood, then the alleles could be AA or AO, thus the allele A frequency is 75%, allele O frequency is 25% for both parents.
Since AA, AO and OA are blood type A, and OO is blood type O, thus their child has 6.25% chance to be blood type O and 93.75% chance to be blood type A.
The +/- is called the rhesus factor, with + being dominant, and - being recessive. So if both parents are -, the kids are always -, otherwise the kids might be + or -.
Blood types are very important when a blood transfusion is necessary. In a blood transfusion, a patient must receive a blood type compatible with his or her own blood type. If the blood types are not compatible, red blood cells will clump together, making clots that can block blood vessels and cause death.
If two different blood types are mixed together, the blood cells may begin to clump together in the blood vessels, causing a potentially fatal situation. Therefore, it is important that blood types be matched before blood transfusions take place. In an emergency, type O blood can be given because it is most likely to be accepted by all blood types. However, there is still a risk involved.