The diagram shows the inheritance of haemophilia, a blood clotting disorder, in three generations of a family - Scottish Highers Human Biology - Question 14 - 2019

To determine the chance of haemophilia for the child of individuals P and Q, we first need to analyze the genotypes of these individuals based on the pedigree diagram provided.

1. Identify the Genotypes:

   • Individual P is a carrier (XH, Xh), where XH is the normal allele and Xh is the allele for haemophilia since he has an affected son (indicated in the diagram).
   • Individual Q is unaffected and is therefore likely homozygous (XH, XH) as both her parents are unaffected and one is affected.

2. Punnett Square Analysis:

   • We can set up a Punnett square to find the probabilities of their offspring’s genotypes:

   	XH	XH
   XH	XHXH	XHXH
   Xh	XHXh	XHXh

3. Resulting Genotypes:

   • The resulting genotypes will be:
     • XHXH (normal female)
     • XHXh (carrier female)
     • XHY (normal male)
     • XhY (affected male)

4. Probabilities:

   • Therefore, the probability of having a child with haemophilia (XhY) is 25% since there is 1 outcome of XhY out of 4 possible combinations.

Based on this analysis, the percentage chance that P and Q's child will have haemophilia is 25%.