DP Biology (first assessment 2025)

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Question 22M.2.SL.TZ1.4b

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Date May 2022 Marks available [Maximum mark: 3] Reference code 22M.2.SL.TZ1.4b
Level SL Paper 2 Time zone TZ1
Command term Explain Question number b Adapted from N/A
b.
[Maximum mark: 3]
22M.2.SL.TZ1.4b
(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.

Syllabus sections