Chapter 11, Problem 21P

The figure that follows shows DNA fingerprint analysis of the genomic DNA from semen associated with a rape (***) and from mouth swabs (somatic cells) of individuals 1–4. This analysis involves the PCR amplification of six SSR loci, each from a different (nonhomologous) chromosome. All PCR primers used are 20 nucleotides long; the primers for each locus have fluorescent tags in a locus-specific color. In the gel, some bands are thicker because relatively more of the corresponding PCR product was obtained. The figure has dots aligned on both sides that you can use to find the critical bands, using the edge of a piece of paper as a guide.

a.	Sperm are haploid, but the semen sample shows two different-sized PCR products for certain loci. How is this possible?
b.	Is any locus on the X chromosome? If so, identify it.
c.	Is any locus on the Y chromosome? If so, which one?
d.	Explain why these results demonstrate that none of the four individuals is the rapist. What pattern would you expect by analyzing mouth swab DNA from the rapist?
e.	Do these results nonetheless provide any information that could help catch the rapist? If so, be as specific as possible.
f.	The two orange bands amplified by PCR from the semen are 200 and 212 bp long. How many tandem repeats of the SSR repeat unit are found in the two alleles of this locus in the rapist’s genomic DNA? (Assume that the PCR products are the shortest possible and that the repeat unit for this locus is TCCG.)