When you look at blood types (aka. blood groups) you might think that it’s determined by just one allele, but surprisingly, it’s actually multiple alleles, that determine the child’s blood group, and it can get quite complex. So first of all, why are there different blood types in the first place?  Well, the A blood group is said to be the first blood type, and existed before humans evolved from hominids, the other blood types are mutations to the gene that were caused in order to combat certain infectious diseases. Later after A type came B type (about 3.5 million years ago), which modified a single sugar on the outside of the red blood cell, and after that (about 2.5 million years ago) came the O gene, which deactivated the sugar completely (this is why in some languages blood type O is called zero or nil). The fact that different blood groups are incompatible however, is a side effect, and wasn’t intended. The reason blood types are incompatible are because  So, how is blood type inherited?  Each parent passes down a 2-piece combination  of either A,B or O genes, A and B genes are dominant, while O is recessive. Because there are 6 different gene combinations from each parent(AA,BB,OO,AB,AO,BO), this makes determining blood type complex, for example, if one parent carries the AA gene, and one the AO gene, there is a chance the offspring obtains either, but they will always show the dominant A type. In the case of AB blood type, the two dominant A and B don’t overcome each other, but rather combine into a type where the blood cell has both antigens on the red blood cell. There is also the Rh factor to consider, which is a protein that covers the red blood cell, this determines the of the blood cell will be positive or negative, positive is dominant over negative, but again there is a chance for either to occur when one of the parents’ genes is Pos/Neg. Overall, the difference in blood type is the antigens in red blood cells, with A containing the A antigen and B antibody in the plasma, B with the B antigen and A antibody, AB with both antigens, and neither antibodies, and O with neither antigens, and both antibodies.

Below is a Punnet square of blood group genes, as well as a diagram of antigens on different blood groups.