Assertion (A): A good example of multiple alleles is ABO blood group…

The correct answer is Option (2) → Both Assertion (A) and Reason (R) are correct but R is not the correct explanation of A.

Assertion (A): A good example of multiple alleles is ABO blood group system. — True. The gene I has three alleles, I^A, I^B and i. More than two alleles govern the same character, so this is a case of multiple alleles.

Reason (R): When I^A and I^B alleles are present together in ABO blood group system, they both express their own types. — True. An I^AI^B person has both A and B types of sugar on the red blood cells. This is co-dominance.

Why R does not explain A? A is about the number of alleles of the gene: there are three of them in the population. R is about how two of these alleles behave when they are together in one person: both express, which is co-dominance. Two co-dominant alleles would express together even if the gene had only two alleles, so R does not show why ABO has multiple alleles. Both statements are true, but R is not the explanation of A.

NCERT Extract:
Co-dominance – “… The gene (I) has three alleles I^A, I^B and i. … But when I^A and I^B are present together they both express their own types of sugars: this is because of co-dominance. Hence red blood cells have both A and B types of sugars.”
Co-dominance – “… Do you realise that the example of ABO blood grouping also provides a good example of multiple alleles? Here you can see that there are more than two, i.e., three alleles, governing the same character.”