Chapter 11, Problem 17P

Sperm samples were taken from two men just beginning to show the effects of Huntington disease. Individual sperm from these samples were analyzed by PCR for the length of the trinucleotide repeat region in the HD gene. In the graphs that follow, the horizontal axes represent the number of CAG repeats in each sperm, and the vertical axes represent the fraction of total sperm of a particular size. The first graph shows the results for a man whose mutant HD allele (as measured in somatic cells) contained 62 CAG repeats; the man whose sperm were analyzed in the second graph had a mutant HD allele with 48 repeats.

 

a.	What is the approximate CAG repeat number in the HD+ alleles from both patients?
b.	Assuming that these results indicate a trend, what can you conclude about the processes that give rise to mutant HD alleles? In what kinds of cells do these processes take place?
c.	How do these results explain why approximately 5–10% of Huntington disease patients have no family history of this condition?
d.	Predict the results if you performed this same PCR analysis on single skin cells from each of these patients instead of single sperm.