Which of the following statements about human blood groups is not true?

Analyze the question: We need the statement about ABO blood groups that is NOT true.

Concept: The ABO blood group is controlled by three alleles: Iᴬ, Iᴮ and i (or O).

• Iᴬ and Iᴮ are both dominant over i (O).
• Iᴬ and Iᴮ are co-dominant with each other. When both are present, the person has blood group AB, and both antigens are expressed.
• i (O) is recessive. Blood group O occurs only when the genotype is ii.

Check each option:

• A. A is dominant over B: False. A and B are co-dominant, so neither is dominant over the other. A person with IᴬIᴮ shows both A and B traits (group AB).
• B. O is recessive: True. O shows only in the homozygous state (ii).
• C. B is dominant over O: True. IᴮIᴮ and Iᴮi both give group B.
• D. A and B are co-dominant: True. This is why group AB exists.

Exam tip: Remember "A and B are equals, O is the one who yields." A and B are co-dominant with each other, and both dominate O. Any option claiming that A dominates B, or B dominates A, is false.