Answer:
There are four blood types: A, B, AB, and O. We won't get into positive and negative Rh factors here.
A and B are codominant, meaning they act like partners when together, forming blood type AB. A and B are both dominant to O. O is completely recessive to all other blood types, meaning that the genotype must contain two O alleles to have Type O Blood.
Type A blood can be AA (h0m0zygous) or AO (heterozygous)
Type B blood can be BB (h0m0zygous) or BO (heterozygous)
Type AB blood is AB (heterozygous)
Type O blood is OO (h0m0zygous)
The various blood types can be combined using Punnett squares to predict probabilities of various blood types.
I hope this answers your question.