Page 107 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

266 Chapter 7 Anatomy and Function of a Gene: Dissection Through Mutation

detected after growth in BrdU). These chromosomes a. Indicate what phenotypes and ratios you would
are caused by high levels of chromosome breakage expect in the F 2 generation of this cross if there is
followed by repair through homologous recombina- one pigment pathway, with orange and black being
tion. In some patients, every cell has many harlequin different intermediates on the way to brown.
chromosomes. In other patients, the majority of cells b. Indicate what phenotypes and ratios you would
have many harlequin chromosomes, but about 10% of expect in the F 2 generation if orange pigment is a
the cells surprisingly have none. product of one pathway, black pigment is the prod-
a. What kinds of events produce the 10% of the uct of another pathway, and brown is the effect of
cells in certain Bloom syndrome patients with no mixing the two pigments in the skin of the snake.
harlequin chromosomes? (Hint: Think about 39. In a certain species of flowering plants with a diploid
recombination.) What does the existence of these genome, four enzymes are involved in the generation
cells lacking harlequin chromosomes say about of flower color. The genes encoding these four en-
the alleles of the Bloom syndrome gene carried zymes are on different chromosomes. The biochemi-
by these patients? cal pathway involved is as follows; the figure shows
b. In what way do Bloom syndrome patients of both that either of two different enzymes is sufficient to
classes reflect the results of complementation tests? convert a blue pigment into a purple pigment.
c. Why does it make sense that the events you described →
in part (a) might occur in Bloom syndrome patients? white → green → blue → purple
d. What is different about the events you described in A true-breeding green-flowered plant is mated with a
part (a) from the events that give rise to harlequin true-breeding blue-flowered plant. All of the plants in
chromosomes? the resultant F 1 generation have purple flowers. F 1
e. Could the events you described in part (a) occur plants are allowed to self-fertilize, yielding an F 2 gen-
during G 1 of the cell cycle? During G 2 ? eration. Show genotypes for P, F 1 , and F 2 plants, and
f. The events that give rise to the cells without harle- indicate which genes specify which biochemical
quin chromosomes are very rare, occurring in less steps. Determine the fraction of F 2 plants with the
than one in a million cell divisions even in Bloom following phenotypes: white flowers, green flowers,
syndrome patients. Surprisingly, however, roughly blue flowers, and purple flowers. Assume the green-
10% of the cells in certain patients lack harlequin flowered parent is mutant in only a single step of
chromosomes. How can these two statements be the pathway.
true simultaneously? 40. The intermediates A, B, C, D, E, and F all occur in
the same biochemical pathway. G is the product of the
−
pathway, and mutants 1 through 7 are all G , meaning
Section 7.5 that they cannot produce substance G. The following
37. The pathway for arginine biosynthesis in Neurospora table shows which intermediates will promote growth
crassa involves several enzymes that produce a series in each of the mutants. Arrange the intermediates in
of intermediates. order of their occurrence in the pathway, and indicate
the step in the pathway at which each mutant strain is
ARG-E ARG-F ARG-G ARG-H blocked. A + in the table indicates that the strain will
N-acetylornithine → ornithine → citrulline → argininosuccinate → arginine
grow if given that substance, an O means lack of growth.
−
−
a. If you did a cross between ARG-E and ARG-H
Neurospora strains, what would be the distribution Supplements
+
−
of Arg and Arg spores within parental ditype and Mutant A B C D E F G
nonparental ditype asci? Give the spore types in
the order in which they would appear in the ascus. 1 + + + + + O +
2 O O O O O O +
b. For each of the spores in your answer to part (a), 3 O + + O + O +
what nutrients could you supply in the media to get 4 O + O O + O +
spore growth?
5 + + + O + O +
38. In corn snakes, the wild-type color is brown. One 6 + + + + + + +
autosomal recessive mutation causes the snake to be 7 O O O O + O +
orange, and another causes the snake to be black. An
orange snake was crossed to a black one, and the F 1 41. In each of the following cross schemes, two true-
offspring were all brown. Assume that all relevant breeding plant strains are crossed to make F 1 plants,
genes are unlinked. all of which have purple flowers. The F 1 plants are