Question

B-blood group man marries A blood group woman. The first child has B-blood group. Genotype of the child is ?

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

Answer 1

The correct option is B $I^B{I}^O$

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${}^B$. A child with “B” blood group from parents with A and B blood group means that irrespective of genotype of parent with B blood group (“I${}^B$I${}^O$" / “I${}^B$I${}^B$”), the parent with A blood group carries at least one “I${}^O$” allele. Thus, genotype of parent with “A” blood group is “I${}^A$I${}^O$”; the child inherited “I${}^O$” allele from parent with “A” blood group and “I${}^B$” allele from parent with “B” blood group. Thus genotype of the child is “I${}^B$I${}^O$”. Since the parent with “A” blood group can not have “I${}^B$” allele; the child can not have “I${}^B$I${}^B$” genotype; option D is wrong. Genotype “I${}^A$I${}^O$”gives “A” blood group (option A is incorrect) and genotype “I${}^A$I${}^B$” gives “AB” blood group (option C is incorrect). The correct answer is B.