He he, very interesting
If only blood characteristics could be easily divided into four types:) The truth is that our typing is decided by a combination of both of our parents. A, B,AB, or O are only phenotypes.
For example, phenotype A can be genotype AA or AO
phenotype B can be genotype BB or BO.
Each parent passes on one of these to their child and the child then has one of four possible combinations of their parents blood type. When you factor in the RH then you have even more combinations and so on.
This is why genotype testing is important to ensure compatibility, because genotyping gives more information than phenotype in performing a crossmatch.
Sharmin