In the cat exists three different blood groups:
A, B and AB.
They are inherited in an autosomal dominant trait with two alleles at the same gene. A is dominant over B.
So cats with blood group A may be either homozygote (A/A) or heterozygote (A/B) Cats with blood group B are always homozygote for B (B/B).
Some cats express both alleles on the erythrocyte membrane. These animals have the blood group AB. There seems to be a third gene that suppresses the dominance of A.
In feline medicine it is absolutely important to know the right blood group before transfusion and breeding.
Cats will show severe signs of incompatibility if a non matching blood group in transfused. For breeders it is essential to know the blood group of the queen and tom, because of a syndrome called neonatal isoerythrolysis (FNI).
If a B-group queen is mated to an A-tom at least 75% of the kittens have blood group A.