Question

A child's blood group is 'O'. Which of the following blood group his parents cannot have?

• A. B and O
• B. A and O
• C. AB
• D. A and B

Answer 1

Answer: (C)

AB

Human blood group inheritance is the example of codominance and multiple alleles. It is governed by three alleles namely I${}^A$, I${}^B$ and I${}^O$. I${}^A$ and I${}^B$ shows codominance, while allele I${}^O$ is recessive to both I${}^A$ and I${}^B$.

Since, each allele can combine with any of three alleles, this gives total six genotypes: I${}^A$ I${}^A$, I${}^A$ I${}^O$, I${}^B$ I${}^B$, I${}^B$ I${}^O$, I${}^A$ I${}^B$ and I${}^O$I${}^O$.

Since, I${}^A$ and I${}^B$ alleles are dominant over I${}^O$ allele; the genotypes I${}^A$ I${}^O$ and I${}^B$ I${}^O$ gives A and B blood group respectively. Genotype of person with 'O' blood group is I${}^O$I${}^O$. This means that each of his parents should have at least one 'I${}^O$' allele. Parents with 'B' (I${}^B$ I${}^O$) and 'O' (I${}^O$ I${}^O$) blood group have at least one I${}^O$ and therefore, can have a child with 'O' blood group. Parents with 'A' (I${}^A$ I${}^O$) and 'O' (I${}^O$ I${}^O$), blood group have at least one I${}^O$ and therefore, can have a child with 'O' blood group. Parents with 'AB' (I${}^A$ I${}^B$) blood group do not have I${}^O$ and therefore, cannot have a child with 'O' blood group. Parents with 'A' (I${}^A$ I${}^O$) and 'B' (I${}^B$ I${}^O$) blood group have at least one I${}^O$ and therefore, can have a child with 'O' blood group.

Thus, the correct answer is option C.