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Date	May 2022	Marks available	3	Reference code	22M.2.SL.TZ1.4
Level	Standard level	Paper	Paper 2	Time zone	Time zone 1
Command term	Explain	Question number	4	Adapted from	N/A

Question

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

State the probability that Edward had hemophilia.

[1]

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

[3]

Markscheme

0.5/50 %;

hemophilia is X-linked/sex-linked/carried on the X chromosome;
females have two X chromosomes
OR
males only have one X chromosome;
hemophilia is caused by a recessive allele;
(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

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 %.

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

Core » Topic 3: Genetics » 3.4 Inheritance

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