Chapter 11, Problem 16P

The trinucleotide repeat region of the Huntington disease locus (HD) in six individuals is amplified by PCR and analyzed by gel electrophoresis as shown in the following figure; the numbers to the right indicate the sizes of the PCR products in bp. Each person whose DNA was analyzed has one affected parent.

a.	Which individuals are most likely to be affected by Huntington disease, and in which of these people is the onset of the disease likely to be earliest?
b.	Which individuals are least likely to be affected by the disease?
c.	Consider the two PCR primers used to amplify the trinucleotide repeat region. If the 5′ end of one of these primers is located 70 nucleotides upstream of the first CAG repeat, what is the maximum distance downstream of the last CAG repeat at which the 5′ end of the other primer could be found? [Assume that the diagram shows the largest HD+ allele possible (that is, 35 CAG repeats).]