Inheritance and Genetics: Question 10
Syllabus 17.4
Human ABO blood group is controlled by a gene with three alleles: IA and IB are codominant with each other, and both are dominant to the recessive allele i. A father has blood group A, but a blood test shows he also carries the recessive i allele. A mother has blood group B, and a blood test shows she also carries the recessive i allele.
(a) State the father's genotype, and explain what is meant by the term codominance, using IA and IB as examples. [2]
(b) Use a genetic diagram to work out the possible genotypes and blood groups of the couple's children, and state the expected ratio of the four possible blood groups. [4]
(c) One of their children has blood group O. Explain how this is possible, given that neither parent has blood group O. [2]
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Worked solution
Part (a): The father’s genotype and codominance
The father has blood group A, which normally could mean he is IAIA or IAi. Since the blood test shows he also carries the recessive i allele, his genotype must be IAi.
Codominance occurs when two different alleles of a gene are both expressed in the phenotype of a heterozygous individual, with neither one masking the other. In the ABO system, IA and IB are codominant: a person with genotype IAIB does not show a “blend” or just one of the two phenotypes, they show blood group AB, meaning both the IA and IB alleles are expressed together.
Part (b): Genetic diagram for the cross
The father is IAi, so he produces two types of sperm in equal numbers: IA and i. The mother is IBi, so she produces two types of egg in equal numbers: IB and i.
| IB | i | |
|---|---|---|
| IA | IAIB | IAi |
| i | IBi | ii |
This gives four equally likely offspring genotypes:
- IAIB. Blood group AB
- IAi. Blood group A
- IBi. Blood group B
- ii. Blood group O
The expected ratio of the four blood groups is 1 AB : 1 A : 1 B : 1 O.
Part (c): Explaining the blood group O child
Neither parent has blood group O themselves: the father shows blood group A because his dominant IA allele masks the recessive i allele he carries, and the mother shows blood group B because her dominant IB allele masks the recessive i allele she carries. Even so, each parent is heterozygous and can pass on either their dominant allele or their hidden i allele when gametes are formed.
For a child to have blood group O, it must have the genotype ii, no dominant allele present at all. This happens when, by chance, both parents pass on their recessive i allele to the same child. Since the diagram in part (b) shows that a quarter of the expected offspring are ii, it is entirely possible for two parents who are themselves blood group A and blood group B to have a child with blood group O.
Final answers
- (a) Father’s genotype: IAi. Codominance: both alleles are expressed together in a heterozygote (e.g. IAIB gives blood group AB), with neither allele masking the other.
- (b) Offspring genotypes IAIB, IAi, IBi, ii in equal numbers; expected ratio 1 AB : 1 A : 1 B : 1 O.
- (c) The child is ii; both parents are heterozygous carriers of the recessive i allele, and both happened to pass it on to this child, so it shows blood group O despite neither parent showing that phenotype.