Part 3 — Applied Problems (30 points)Three problems, 10 points each.

(a) A dividing skin cell is exposed to ultraviolet light. Two adjacent thymines on the same strand become covalently joined to each other. (b) In a different cell, replication has just finished and a G is found paired with a T in the newly synthesised strand; the polymerase’s proofreading activity did not catch it.

Explain what changes about the decision when PGD is used to select a trait rather than to prevent a disease. Then name one practical factor, unrelated to the ethics of the trait itself, that limits how widely PGD is used. (2 pts)

Part 2 — Data Interpretation (24 points)Three problems

Two unaffected parents, I-1 and I-2, have three children: II-1 (affected), II-2 (unaffected), and II-3 (unaffected). The disorder is rare and single-gene. Using A (dominant, normal) and a (recessive, disease), what is II-1’s most probable genotype?

In the same family, II-3 (unaffected) has children with II-4 (unaffected, married into the family, assumed not a carrier): their son III-1 is affected and their daughter III-2 is unaffected. Considering the whole family (two unaffected parents I-1/I-2 producing an affected child II-1, and unaffected II-3 producing an affected son with a non-carrier partner), what is the overall mode of inheritance for this disorder?

Using your expected genotype counts, which of the following confirms they are consistent with the Hardy–Weinberg model applied to this population? (2 pts)

The pedigree below shows the occurrence of a single-gene condition in one family. Assume the condition is fully penetrant and that no individual marrying into the family carries the allele.

The pathway that corrects the G–T mismatch described in (b), after replication, is: (2 pts)

Write out the patient’s sequence as read from this gel, 5′ → 3′. Then state which end of the gel corresponds to the 5′ end of the sequence, and explain why in terms of fragment size and migration. (3 pts)