DP Biology (last assessment 2024)

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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

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

(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