Dominance hierarchy in ABO blood groups

Steps:

• Identify alleles: I^A and I^B dominant over I^O; I^A and I^B codominant.
• Define homozygous dominant: Both alleles identical and dominant (e.g., I^A I^A).
• Evaluate options: A heterozygous (I^A dominant, I^O recessive); B homozygous I^A; C heterozygous codominant; D homozygous recessive.
• Select matching genotype: B fits homozygous dominant for I^A.

Why B is correct:

• I^A I^A has two identical dominant I^A alleles, expressing type A blood fully.

Why the others are wrong:

• A: Heterozygous; one dominant, one recessive allele.
• C: Heterozygous codominant; different dominant alleles produce type AB.
• D: Homozygous recessive; both I^O alleles produce type O.

Final answer: B