DP Biology (first assessment 2025)

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Question 22M.2.SL.TZ2.b

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Date May 2022 Marks available [Maximum mark: 3] Reference code 22M.2.SL.TZ2.b
Level SL Paper 2 Time zone TZ2
Command term Determine Question number b Adapted from N/A
b.
[Maximum mark: 3]
22M.2.SL.TZ2.b

A family has a history of hemophilia that is caused by a sex-linked recessive allele. A woman from this family is a carrier and marries a man who does not have the allele. Showing your working, determine the probability of their children having the disease.

[3]

Markscheme
  1. alleles clearly labelled for both parents (in a Punnett square or other format);
  2. correct genotypes for all four possibilities for children;
  3. phenotypes of the children: the sons would have a 50 % chance of having hemophilia and the daughters would have 0 % chance of having hemophilia/50 % chance of being a carrier;

Sex needs to be mentioned for mpc but info can be taken from the Punnett square.

Examiners report

A standard sex -linked genetics question. It was expected that an indication that half of the boys would suffer from haemophilia, not just a ‘25 % possibility’.

Syllabus sections