Question

If one child in a family has blood group AB and another child has O, what will be the genotypes of their parents?

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

Answer 1

Answer: (C)

The IA allele gives type A, IB gives type B, and IO gives type O. As both IA and IB are dominant over IO, only IO/IO people have type O blood. Individuals with IA/IA or IAIO have type A blood, and individuals with IB/IB or IB/IO have type B. IA/IB people have both phenotypes because A and B express a special dominance relationship: codominance, which means that type A and B parents can have an AB child. A couple with type A and type B can also have a type O child if they are both heterozygous (IB/IO,IA/IO).

Therefore, the correct answer is option C.