How do you thinkthe alleles of human ABO blood group combine to make four phenotypes.?

Answer 1

A and B are codominant, while O is just the absence of both. So OO genotype yields O phenotype, AA or AO yield A, BB or BO yield B, and AB yields AB.

Answer 2

The three alleles are:
Ia
Ib
i

The four Phenotypes are A, B, AB, and O

Ia Ia = A
Ia i = A

Ib Ib = B
Ib i = B

Ia Ib = AB

i i = O