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Challenge Questions

This page contains further questions on the subject of inherited vision impairment and are designed to stimulate more in-depth conversation and learning. These questions support students to think and write at the level of synthesis and understanding required for Merit and Excellence discussions and above.


1. (a) Explain what a Barr Body is and why they are only found in females

(b) How could the presence of Barr Bodies affect the phenotype of a female who is: 

  • Heterozygous for an x-linked gene?
  • Homozygous for an x-linked gene?

(c) All the females in the family who are affected are heterozygous for the mutation. Within the females in the whanau, there is variation in the severity of the vision impairment. What biological explanation could account for this variation in the heterozygous female phenotype?


2. Use Fig. 2 below to answer the following question: What evidence from the family tree suggests that this is an x-linked condition?




3. Use Fig. 5 below to complete the following tasks and answer the questions:


a) Find the pattern 2 1 4 1 1 3 2 2 on female 5 and 6 in Generation V.

b) Highlight every place in the family tree where this pattern is seen.

c) Crossing over has occurred in person 17 and person 48. Draw yourself a diagram to show this cross over.

d) Compare person 16 in generation VII (affected) with person 17 in generation VI (unaffected). What does the difference between the microsatellite pattern for these two individuals tell us about where on the chromosome the mutation is? Hint: Look at the sequence, “2 2 6”.

e) Compare person 45 in generation VII (unaffected) with person 48 in generation VII (affected). The difference between these two individuals confirmed for the scientists that the mutation must be in the middle section of the chromosome where the satellite pattern “1 1 3 2” was found. What evidence did they use to come to this conclusion?


  • The roman numerals (IV, V, VI, VII) on the left represent the generations
  • The symbols are the same as in the tree in Fig 2.
  • The numbers beside the symbols represent a person
  • The numbers in columns under the symbols represent different alleles of each of the microsatellite markers that were tested