Page 102 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

Problems 261

8. The results of the fluctuation test (Fig. 7.5) were in- E and B genes listed in Table 3.3 (see also Figs. 3.12
terpreted to mean that different numbers of mutant and 3.13). (Also note that the black marking is
bacteria preexisted in each of the 11 culture tubes be- not a mask.)
cause the mutations arose spontaneously at different
times during the growth of each culture. However, an-
other possibility is that the differences in the number
of colonies on the plates are simply due to differences
in the ability of the petri plates to support the growth
of colonies. For example, perhaps the selective agent
or the nutrients in the media were not evenly distrib-
uted in the molten agar poured into the petri dishes.
What experiment could you do to determine whether
or not differences in the petri plates were a factor in
the experiment?

9. The following pedigree shows the inheritance of a
completely penetrant, dominant trait called amelogen-
esis imperfecta that affects the structure and integrity
of the teeth. DNA analysis of blood obtained from af-
fected individuals III-1 and III-2 shows the presence
of the same disease-causing mutation in one of the
two copies of an autosomal gene called ENAM that is
not seen in DNA from the blood of any of the parents © Sally MacBurney
in generation II. Explain this result, citing Fig. 4.19
and Fig. 7.5. Do you think this type of inheritance Section 7.2
pattern is rare or common?
12. Remember that Balancer chromosomes prevent the
recovery of recombinant chromosomes between the
Balancer and its normal homolog. Why was the
I Balancer X chromosome crucial to the design of
1 2 Muller’s experiment (Fig. 7.13)? (Hint: The best way
II to answer this question is to consider what the experi-
1 2 3 mental results would have been without the Balancer.)
III
1 2 13. Figure 7.14 shows examples of base substitutions
induced by the mutagens 5-bromouracil, hydroxyl-
amine, ethylmethane sulfonate, and nitrous acid.
10. Autism is a neurological disorder thought to be Which of these mutagens cause transitions, and
caused by mutant alleles of one or more genes. which cause transversions?
Scientists had been wondering why the number of 14. Figure 7.14a shows the mutagen 5-bromouracil
children diagnosed as autistic increased dramatically (5-BU), which can resemble either T or C depend-
in a decade, from 1 in 500 in 2002 to 1 in 88 in 2012. ing on its tautomeric state. The figure first shows
Researchers now think that they might have found at 5-BU incorporated into DNA as the T-like tautomer,
least part of the answer: Men are fathering children at but then it changes its state to the C-like tautomer
later and later ages. A paper published in the journal during the next round of DNA replication. The re-
Nature in 2012 showed a correlation between paternal sult was a T:A→C:G substitution. Suppose that the
age and the incidence of autism; the age of the mother tautomeric states of 5-BU during the two rounds of
was not a factor. How does this observation provide a replication were reversed. What kind of mutation
possible explanation for the apparent increase in the would result?
rate of autism?
15. So-called two-way mutagens can induce both a par-
11. Like the yellow Labrador retrievers featured in ticular mutation and (when added subsequently to
Chapter 3, golden retrievers are usually solid yellow. cells whose chromosomes carry this mutation) a
The golden retriever shown has an extremely rare reversion of the mutation that restores the original
black marking on its face. Explain the genetic basis DNA sequence. In contrast, one-way mutagens can
for the appearance of this dog. Consider only the induce mutations but not exact reversions of these