DP Biology (first assessment 2025)

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Question 22M.2.SL.TZ1.4

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Date May 2022 Marks available [Maximum mark: 4] Reference code 22M.2.SL.TZ1.4
Level SL Paper 2 Time zone TZ1
Command term Explain, State Question number 4 Adapted from N/A
4.
[Maximum mark: 4]
22M.2.SL.TZ1.4

The pedigree chart shows the incidence of hemophilia in some of the descendants of Queen Victoria.

(a)

State the probability that Edward had hemophilia.

[1]

Markscheme

0.5/50 %;

Examiners report

Approximately 50 % of the candidates were correct with their answer of 50 %. It was expected that candidates would recognize that Edward is male because he has a box rather than circle in the pedigree. Then from there, they would calculate the 50 %.

(b)

Explain the reasons for none of the females in the pedigree chart having hemophilia.

[3]

Markscheme
  1. hemophilia is X-linked/sex-linked/carried on the X chromosome;
  2. females have two X chromosomes
    OR
    males only have one X chromosome;
  3. hemophilia is caused by a recessive allele;
  4. (trait) must be on both alleles to be expressed
    OR
    females would require the allele on both X chromosomes to have the disease
    OR
    females can be carriers when allele is only on one chromosome;
Examiners report

There were many good answers for this question showing understanding of haemophilia being sex linked and recessive Quite a number then failed to get the third mark by being too vague in their explanations. Some candidates thought that hemophilia is carried on Y chromosome; many understand that females have two X chromosomes; talk about females as carriers was often seen, but without any details to expand the answer; or that males are most likely to have hemophilia but, again, no details; for the most part answers showed shallow understanding.

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