Inheritance and Genetics: Question 9
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 woman with blood group AB (genotype IAIB) has children with a man with blood group O (genotype ii).
Which statement correctly gives the possible blood groups of their children?
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Worked solution
Step 1: Set up the genetic diagram
The mother, genotype IAIB, produces two types of egg in equal numbers: one carrying IA and one carrying IB. The father, genotype ii, can only produce sperm carrying i, since both of his alleles are i.
| IA | IB | |
|---|---|---|
| i | IAi | IBi |
Step 2: Read off the offspring genotypes and phenotypes
Every child inherits i from the father, combined with either IA or IB from the mother, in equal proportions:
- IAi. Blood group A (since IA is dominant to i)
- IBi. Blood group B (since IB is dominant to i)
No child can be blood group AB, because that requires inheriting both IA and IB, but here, only one of these two alleles is available (from the mother), and the father can only contribute i. No child can be blood group O either, because that requires ii, but every child receives at least one dominant allele (IA or IB) from the mother. The expected ratio is 1 blood group A : 1 blood group B.
Why the other options are wrong
- B: this wrongly assumes both of the mother’s alleles are passed to every child. In fact, each child inherits only one of IA or IB from the mother (not both), so AB is impossible here. Codominance only produces the AB phenotype in an individual who inherits one of each allele.
- C: this wrongly treats IA as dominant to IB and applies a 3:1 ratio, which belongs to an ordinary heterozygous-by-heterozygous cross for a single dominant/recessive gene, not this codominant, three-allele system.
- D: this wrongly assumes the father’s recessive allele masks the mother’s alleles in the children. A recessive allele can only mask a dominant allele if an individual is homozygous recessive; here, every child receives a dominant allele from the mother, so none can be blood group O.
Final answer
Every child inherits i from the father and either IA or IB from the mother, giving blood group A or blood group B in a 1:1 ratio, with no AB or O children possible, option A.