Question: 19) The figure below shows DNA fingerprint analysis of the genomic DNA from semen associated with…

19) The figure below 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 microsatellite
repeating 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 repeating unit for this locus is
TCCG.)