Inheritance and Genetics: Question 5
Syllabus 17.4
Haemophilia is a blood-clotting disorder caused by a recessive allele, h, carried on the X chromosome. The dominant allele, H, allows normal blood clotting. A man who has haemophilia (genotype XhY) has children with a woman who has normal blood clotting but is a carrier of the h allele (genotype XHXh).
Which statement correctly gives the chance that a child of this couple is an unaffected carrier daughter, and correctly identifies which parent contributes the h allele she carries?
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Worked solution
Step 1: Set up the genetic diagram
The father is XhY, so he produces two types of sperm in equal numbers: one carrying his X chromosome (with the h allele) and one carrying his Y chromosome. The mother is XHXh, so she produces two types of egg in equal numbers: one carrying the X chromosome with the H allele, and one carrying the X chromosome with the h allele.
Combining these gametes gives four equally likely offspring:
| Xh | Y | |
|---|---|---|
| XH | XHXh | XHY |
| Xh | XhXh | XhY |
Step 2: Read off the genotypes and phenotypes
- XHXh. Daughter, unaffected, carrier of h
- XHY. Son, unaffected
- XhXh. Daughter, has haemophilia
- XhY. Son, has haemophilia
Only one of the four equally likely outcomes, XHXh, is an unaffected carrier daughter, so the overall chance of this outcome is 1 in 4.
Step 3: Trace where the carrier daughter’s alleles came from
Every daughter must inherit an X chromosome from each parent. The father is XhY, so the only X chromosome he has to give carries h. He therefore contributes h to every daughter he has, with no exception. The mother is XHXh, so she contributes either H or Xh with equal chance. A daughter who receives H from her mother, combined with the h she is guaranteed to receive from her father, becomes the unaffected carrier XHXh. So the h allele in this carrier daughter came from her father, and the H allele came from her mother.
Why the other options are wrong
- B: this gives the correct 1 in 4 chance, but wrongly attributes the h allele to the mother. Fathers do pass an X chromosome to every daughter, and here that X chromosome carries h, so it is the father who is the source of the daughter’s h allele, not the mother.
- C: this correctly identifies the father as the source of the h allele, but overstates the probability. A 1 in 2 chance of being a carrier applies only among daughters; since only half of all children are daughters, the overall chance across all four equally likely offspring is 1 in 4, not 1 in 2.
- D: this misunderstands what a carrier is. A son has only one X chromosome, so he is always either affected (XhY) or unaffected (XHY). He can never be a carrier. Only a daughter, with two X chromosomes, can be an unaffected carrier (XHXh).
Final answer
The genetic diagram gives a 1 in 4 overall chance that a child is an unaffected carrier daughter, and her h allele must come from her father, since his only X chromosome carries h and he passes it to every daughter, option A.