Question

A man with blood group B marries a female with blood group A and their first child is having blood B. What is the genotype of child

• A. $I^A{I}^B$
• B. $I^A{I}^O$
• C. $I^B{I}^O$
• D. $I^B{I}^B$

Answer 1

Answer: (C)

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${}^B$ show codominance while allele I${}^O$ is recessive to both I${}^A$ and I${}^B$. This gives total six genotypes and four phenotypes- A (I${}^A$ I${}^A$ and I${}^A$ I${}^O$), B (I${}^B$ I${}^B$ and I${}^B$ I${}^O$) , AB (I${}^A$ I${}^B$) and O (I${}^O$ I${}^O$). Since the mother has A blood group, the child can not be homozygous for B blood group. The reason is that he will receive one I${}^B$ allele from father with B blood group but the mother with A blood group can transmit only I${}^O$ allele for the child to have B blood group. Hence the genotype of child will be I${}^B$ I${}^O$. Option D is incorrect and C is correct. Genotypes in options A and B gives AB and A blood groups respectively which makes them wrong.