Blood types A, B, AB, and O are the result of codominant inheritance. This means that:

* Multiple alleles exist for the gene that determines blood type. In this case, there are three alleles: A, B, and O.

* A and B alleles are codominant. This means that when both A and B alleles are present, both are expressed, resulting in the AB blood type.

* The O allele is recessive. This means that it is only expressed when two O alleles are present.

Here's a breakdown of how the different blood types are inherited:

* Blood type A: Can be inherited from AA or AO genotypes

* Blood type B: Can be inherited from BB or BO genotypes

* Blood type AB: Can only be inherited from AB genotype

* Blood type O: Can only be inherited from OO genotype

This complex inheritance pattern is what creates the variety of blood types we see in humans.