The number of alleles controlling blood groups in humans

Analyze the question: It asks how many alleles control the ABO blood groups in humans.

Explain the concept: A gene can have different forms called alleles. The ABO blood group system is controlled by one gene with three alleles:

• I^A (produces antigen A)
• I^B (produces antigen B)
• i (or I^O) (produces no antigen)

I^A and I^B are codominant, and both are dominant over i.

Step-by-step solution:

1. A person inherits only two alleles, one from each parent.
2. The population has three different alleles for the gene.
3. These combine into six genotypes, giving four phenotypes: • I^A I^A or I^A i → Group A • I^B I^B or I^B i → Group B • I^A I^B → Group AB • ii → Group O

So the number of alleles controlling blood groups is 3. Do not confuse this with the number of blood groups (4) or the number of alleles one person carries (2). Option D (2) is the number of alleles an individual has, and option B (4) is the number of blood groups.

Tip: Remember "3 alleles, 6 genotypes, 4 phenotypes" for ABO. Exam questions often try to trick you with 4 (groups) or 2 (alleles per person), so read carefully whether the question asks about alleles, genotypes or phenotypes.