Inheritance: Question 3
Syllabus 16.2
The ABO blood group system in humans is controlled by a single gene with three alleles: IA and IB, which are codominant with each other, and i, which is recessive to both IA and IB. A person's blood group depends on their genotype as follows: genotype IAIA or IAi gives blood group A; genotype IBIB or IBi gives blood group B; genotype IAIB gives blood group AB; and genotype ii gives blood group O.
A man with blood group A has the genotype IAi. A woman with blood group B has the genotype IBi. They have children together.
(a) State which two alleles of this gene show a codominant relationship with each other, and which allele is recessive to both of the others. [2] (b) Construct a genetic diagram for a cross between this man and this woman, and use it to determine the possible blood groups of their children and the expected ratio between them. [4] (c) Explain how it is possible for this couple, who have blood groups A and B themselves, to have a child with blood group O. [2]
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Worked solution
Part (a): Identifying the codominant and recessive alleles
This gene has three alleles: IA, IB and i. The genotype-to-phenotype rules given in the question show that when both IA and IB are present together (genotype IAIB), both are expressed and the phenotype is blood group AB, rather than one allele masking the other. This is the defining feature of codominance, so IA and IB are codominant with each other.
The allele i never appears on its own in a phenotype unless paired with another i: genotype IAi still gives blood group A, and IBi still gives blood group B, meaning i is masked whenever a IA or IB allele is present. Only the homozygous genotype ii produces blood group O. So i is recessive to both IA and IB.
Part (b): Genetic diagram for the cross
Father: IAi (blood group A) Mother: IBi (blood group B)
Gametes from father: IA, i Gametes from mother: IB, i
| IA (from father) | i (from father) | |
|---|---|---|
| IB (from mother) | IAIB | IBi |
| i (from mother) | IAi | ii |
Offspring genotypes, each with an equal 1-in-4 probability: IAIB, IAi, IBi, ii.
Converting each genotype to a phenotype using the rules given in the question:
- IAIB → blood group AB
- IAi → blood group A
- IBi → blood group B
- ii → blood group O
So the children are expected to show all four blood groups (AB, A, B and O) in the ratio 1 : 1 : 1 : 1.
Part (c): Explaining a blood group O child
Blood group O only arises from the homozygous recessive genotype, ii. Even though the father has blood group A and the mother has blood group B, neither of their genotypes is homozygous: the father is IAi (not IAIA) and the mother is IBi (not IBIB). Each of them therefore carries one “hidden” i allele alongside their dominant-acting allele, without this affecting their own phenotype.
If a child happens to inherit the i allele from the father and the i allele from the mother (as shown in the bottom-right cell of the Punnett square in part (b)), that child’s genotype will be ii. Since i is recessive to both IA and IB, and there is no IA or IB allele present to be expressed, the child will have blood group O, a phenotype not shown by either parent.
Final answers
- (a) IA and IB are codominant; i is recessive to both.
- (b) The cross IAi x IBi gives blood groups AB, A, B and O in a 1 : 1 : 1 : 1 ratio.
- (c) A blood group O child is possible because both parents are heterozygous carriers of the recessive i allele, and a child inheriting i from both parents is homozygous recessive (ii), showing blood group O even though neither parent does.