User interface language: English | Español

Date May 2022 Marks available 1 Reference code 22M.2.SL.TZ1.4
Level Standard level Paper Paper 2 Time zone Time zone 1
Command term State 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]
a.

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

[3]
b.

Markscheme

0.5/50 %;

a.
  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;
b.

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 %.

a.

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.

b.

Syllabus sections

Core » Topic 3: Genetics » 3.4 Inheritance
Show 57 related questions
Core » Topic 3: Genetics
Core

View options