6.10 Blood groups and sex-linked disorders
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What is meant by codominance? Use blood groups as an example.
Mark scheme
- both alleles are expressed (in the phenotype) (1)
- I^{A} I^{B} gives blood group AB (1)
Model answer
Codominance is when both alleles are expressed. A person with I^{A} and I^{B} has blood group AB.
Give the possible genotypes of a person with blood group A.
Mark scheme
- I^{A} I^{A} (1)
- I^{A} I^{O} (1)
Model answer
I^{A} I^{A} or I^{A} I^{O}.
A father is blood group A (I^{A} I^{O}) and a mother is blood group B (I^{B} I^{O}).
Use a Punnett square to find the blood groups their children could have.
Mark scheme
- gametes I^{A}, I^{O} and I^{B}, I^{O} (1)
- offspring I^{A} I^{B}, I^{A} I^{O}, I^{B} I^{O}, I^{O} I^{O} (1)
- AB, A, B, O (1)
Model answer
Gametes I^{A} and I^{O} from the father, I^{B} and I^{O} from the mother. Offspring I^{A} I^{B} (AB), I^{A} I^{O} (A), I^{B} I^{O} (B) and I^{O} I^{O} (O): any of the four blood groups.
Explain why red-green colour blindness is more common in males than in females.
Mark scheme
- the allele is on the X chromosome (1)
- males have only one X chromosome (1)
- so one recessive allele causes colour blindness; females need two (1)
Model answer
The allele is recessive and carried on the X chromosome. Males have only one X, so a single recessive allele makes them colour blind, while females need two copies.
A woman who is a carrier for haemophilia (X^{H} X^{h}) has children with an unaffected man (X^{H} Y).
What is the probability that a son is affected?
Mark scheme
- sons are X^{H} Y or X^{h} Y (1)
- 1 in 2 / 50% (1)
Model answer
Each son gets either X^{H} or X^{h} from his mother, so the probability that a son has haemophilia is 50%.
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