Question

A child with O blood group cannot have parents with the genotype
(A) $I^O$ $I^O$
(B) $I^A$ $I^A$
(C) $I^A$ $I^O$
(D) $I^B$ $I^O$

Answer 1

ABO blood groups in human beings are controlled by the gene I. The gene I has three alleles namely, $I^A$, $I^B$ and $I^O$ (i), which stands as an example of multiple alleles. Since humans are diploid in nature, only two of the three alleles will be found, which is respons$I^B$le for the blood group of the individual.

Complete answer:
ABO blood grouping is discovered by Karl Landsteiner. ABO blood groups in humans are controlled by the gene I. The gene possesses three alleles, $I^A$, $I^B$ and $I^O$ (i). The alleles $I^A$ and $I^B$ produce a slightly different form of the sugar while allele $I^O$ does not produce any sugar. Since humans are diploid in nature, only two of the three alleles will be found.

The genotypes $I^A$ $I^A$ and $I^A$ $I^O$ give rise to A blood group. The genotype $I^B$ $I^B$ and $I^B$ $I^O$ gives rise to B blood groups. When both the alleles $I^A$ and $I^B$ are present, the person possesses AB blood group. While, when both the alleles are $I^O$, the blood group of the person is O. So, it is understood that for the child to have O blood group, both of the parents should have at least one $I^O$ allele in their genotype.

Understanding the options one by one,
(A) $I^O$ $I^O$ – Since both the alleles are $I^O$, there are chances of the child to be born with an O blood group. This is not the correct answer.
(B) $I^A$ $I^A$ – In this case, one of the parents possess both $I^A$ allele, and so, even if the other parent possesses the allele $I^O$, there are no chances for their child to have O blood group. Hence, this is the correct answer.
(C) $I^A$ $I^O$ – Out of two alleles, one is $I^O$. This when combined with the $I^O$ allele of the other parent, there are possibilities for their child to have O blood group. This is not the correct answer.
(D) $I^B$ $I^O$ – Since, out of the two alleles, one is $I^O$, there are poss$I^B$ilities for their child to possess O blood group, if the other parent has at least one $I^O$ allele.

Therefore, Option (B) is the correct answer.

Note: It is important to note that ABO blood grouping is an example of multiple alleles. For the child to possess an O blood group, both of the parents should have at least a single $I^O$ allele in their genotype. The child with O blood group is poss$I^B$le even if the parents possess A and B blood groups, given that their genotype should be $I^A$ $I^O$ and $I^B$ $I^O$ respectively.