The figure below represents the analysis of PCR products amplified using primers

Question

The figure below represents the analysis of PCR products amplified using primers for the region of the HD gene mutated in Huntington’s disease. The mutation that causes Huntington’s disease is an amplification, or expansion, of a CAG repeat within the gene.

The father, represented by the first black square, was diagnosed with Huntington’s disease when he was 40 years old. Among his 9 children, 6 (3, 5, 7, 8, 10, 11) have Huntington’s disease. The number above the band from the PCR fragment represents the age when symptoms first started.

a. Predict whether or not children 4, 6, and 9 will develop Huntington’s disease and, if you answer yes, estimate the age of onset. For child 4, explain why you do or do not think the child will develop Huntington’s disease.

b. Describe how the age of onset correlates with the number of CAG repeats in the allele.

Explanation / Answer

a) Among children 4, 6 and 9 there are chances that child 6 develops a Huntington's disease.
In Huntington's disease number of CAG repeats is inversely proportional to the age of onset i.e. more the number of repeats the lower the age of onset of symptoms. So by seeing the CAG repeats of child 6 (which is similar to father) it can be predicted that the onset of symptoms could be at age around 40.
For child 4, it is not expected that child 4 develops the disease because there is clearly no band for CAG repeats in child 4, showing that its HD gene is not mutated.

b) As described in the previous answer the age of onset of symptoms is inversely related to number of CAG repeats in the gene i.e. more the number of repeats the lower the age of onset of symptoms.

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