The above calculator uses the blood group of the biological mother and father and the rhesus factors to determine the potential blood groups the subject can have.
This is based on theoretical principles first introduced in the early 20th century, when Karl Landsteiner noticed that red blood cells have different molecules on their surface. This provided the basis of a blood type classification. Type A has the A kind of molecules on their surface, type B has B molecules and type O has neither molecule.
Research carried out in 1910 and 1911 in Germany, by Ludwik Hirszfeld and Emil von Dungern, showed that the ABO blood types are inherited.
The ABO blood group system is determined by the ABO gene. The four blood groups, determined by this system: A, B, AB and O, arise from inheriting one or more of the alleles: A, B or O.
Each biological parent donates one of their two ABO alleles to their child. For example, a mother with blood type B can only pass a B allele, whilst a mother with AB can pass an allele that is either A or B. The two alleles form the blood type of the offspring.
| Parental blood groups | Child's blood group |
| O and O | O |
| O and A | O or A |
| O and B | O or B |
| O and AB | A or B |
| A and A | A or O |
| A and B | O or A or B or AB |
| A and AB | A or B or AB |
| B and B | O or B |
| B and AB | B or A or AB |
| AB and AB | A or B or AB |
Blood types are 100% genetically inherited. It was observed however, an environmental influence on which types are passed on more frequently, a process of natural selection.
According to the American Red Cross the rarest blood type is AB(-). B(-) and O(-) are also very rare, each accounting for less than 5% of the world's population.
With the discovery of the Rhesus system that classifies blood into Rh positive and Rh negative, nowadays there are eight different blood types amongst the population.
The Rh system depends on the existence of two genes RHD and RHCE. Rh positivity or Rh negativity marks the presence or absence of the dominant Rh gene. The following table introduces the possible parental combinations and potential child Rh.
| Parental Rh type | Child's Rh type |
| Positive and Positive | Positive or Negative |
| Positive and Negative | Positive or Negative |
| Negative and Negative | Negative |
Transfusion of integral blood or plasma is governed by rules, which depend on the antigens present on the erythrocytes.
The universal red cell donor has Type O negative blood type. The universal plasma donor has Type AB positive blood type.
The table below summarizes the transfusion rules amongst all 8 blood types (Rh factor included):
| Blood Type | Can donate blood to | Can receive blood from |
| A+ | A+ AB+ | A+ A- O+ O- |
| O+ | O+ A+ B+ AB+ | O+ O- |
| B+ | B+ AB+ | B+ B- O+ O- |
| AB+ | AB+ | Everyone |
| A- | A+ A- AB+ AB- | A- O- |
| O- | Everyone | O- |
| B- | B+ B- AB+ AB- | B- O- |
| AB- | AB+ AB- | AB- A- B- O- |
The blood type is one of the determinations that can be done on a sample of blood, e.g. that taken for a complete blood count.
A drop of the blood to be tested is mixed with a serum containing anti- A antibodies. Another drop is mixed with serum containing anti- B antibodies. A reaction of agglutination will then happen with one of the drops.
If the blood in contact with the anti-A serum agglutinates, but the blood in contact with the anti-B serum doesn’t, the blood type is A.
If the opposite happens (i.e. the blood in contact with anti-B serum agglutinates, but not the other) the blood type is B.
If both drops of blood agglutinate, the blood is type AB. If none of the drops agglutinates, the blood is type O.
1. Letsky EA, Leck I, Bowman JM. (2000) Chapter 12: Rhesus and other haemolytic diseases. Antenatal & neonatal screening (2nd ed.) Oxford University Press.
2. Maton A, Hopkins J, McLaughlin CW, Johnson S, Quon Warner M, LaHart D, Wright JD. (1993) Human Biology and Health. Englewood Cliffs NJ: Prentice Hall.