Question

If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents?

Answer 1

Answer:

If the father’s blood group is O, he has the genotype

I

O

I

O

or (ii) . The mother has blood type AB, which means that her genotype would be

I

A

I

B

. The possible combinations of these two groups would result in children having genotypes

I

A

I

O

o

r

I

B

I

O

, which would give them blood groups A or B respectively. Thus, blood group O cannot appear among children if one of the parents has the group AB.

Explanation:

If the father’s blood group is O, he has the genotype

I

O

I

O

or (ii) . The mother has blood type AB, which means that her genotype would be

I

A

I

B

. The possible combinations of these two groups would result in children having genotypes

I

A

I

O

o

r

I

B

I

O

, which would give them blood groups A or B respectively. Thus, blood group O cannot appear among children if one of the parents has the group AB.

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).

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