Page 187 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

Problems 179

crossover, occurs between them in every f. Why in this problem can you obtain gene ↔ centro-
meiosis?) mere distances from the crosses in the table, even
b. Can a gene and the centromere be unlinked? though the data are not presented as ordered octads?
Explain.
c. Suppose in an ordered tetrad analysis you observe Section 5.6
an RF of 30% between a gene and the centromere. 47. A single yeast cell placed on a solid agar will divide
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Given your answer to part (a), do think that 30 m.u. mitotically to produce a colony of about 10 cells. A
is an accurate estimate of the distance between the haploid yeast cell that has a mutation in the ade2 gene
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gene and the centromere? will produce a red colony; an ade2 colony will be
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46. A research group has selected three independent Trp white. Some of the colonies formed from diploid
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haploid strains of Neurospora, each of which cannot yeast cells with a genotype of ade2 / ade2 will
grow in the absence of the amino acid tryptophan. contain sectors of red within a white colony.
They first mated these three strains with a wild-type a. How would you explain these sectors?
strain of opposite mating type, and then they analyzed b. Although the white colonies are roughly the same
the resultant octads. For all three matings, two of the size, the red sectors within some of the white
four spore pairs in every octad could grow on minimal colonies vary markedly in size. Why? Do you
medium (that is, in the absence of tryptophan), while expect the majority of the red sectors to be relatively
the other two spore pairs were unable to grow on this large or relatively small?
minimal medium. 48. Figure 5.29 shows mitotic recombination leading to
a. What can you conclude from this result? single spots or twin spots occurring in the G 2 stage of
In the matings of mutant strains 1 and 2 with wild type, the cell cycle (after the chromosomes have replicated).
one of the two topmost pairs in some octads had spores However, because it usually is initiated by rare, random
that could grow on minimal medium while the other of events of chromosome breakage, mitotic recombina-
the two topmost pairs in the same octads had spores that tion can also take place in G 1 , prior to S phase. Redraw
could not grow on minimal medium. In the mating of Fig. 5.29 with mitotic recombination taking place in
mutant strain 3 with wild type, either all the spores in G 1 rather than G 2 , and demonstrate why any such
the two topmost pairs could grow on minimal medium event could not yield single spots or twin spots.
or all could not grow on minimal medium. 49. A diploid strain of yeast has a wild-type phenotype
b. What can you conclude from this result? but the following genotype:
The researchers next prepared two separate cultures of b a c leth d e
each mutant strain; one of these cultures was of mating
type A and the other of mating type a. They mated
these strains in pairwise fashion, dissected the resultant b + a + c + leth + d + e +
octads, and determined how many of the individual a, b, c, d, and e all represent recessive alleles that yield
spores could grow on minimal medium. The results a visible phenotype, and leth represents a recessive
are shown here.
lethal mutation. All genes are on the same chromosome,
and a is very tightly linked to the centromere (indicated
% of octads with x number of spores viable
Mating on minimal medium by a small circle). Which of the following phenotypes
could be found in sectors resulting from mitotic recom-
x = 0 2 4 6 8 bination in this cell? (1) a; (2) b; (3) c; (4) d; (5) e;
1 × 2 78 22 0 0 0 (6) b e; (7) c d; (8) c d e; (9) d e; (10) a b. Assume that
1 × 3 46 6 48 0 0
double mitotic crossovers are too rare to be observed.
2 × 3 42 16 42 0 0
50. In Drosophila, the yellow (y) gene is near the telomere
c. For each of the three matings in the table, how of the long arm of the acrocentric X chromosome,
many of the 100 octads are PD? NPD? T? while the singed (sn) gene is located near the middle of
d. Draw a genetic map explaining all of the preceding the same X chromosome arm. On the wings of female
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data. Assume that the sample sizes are sufficiently flies of genotype y sn / y sn , you can very rarely find
small that none of the octads are the result of patches of yellow tissue within which a small subset of
double crossovers. cells also have singed bristles.
e. Although this problem describes crosses in a. How can you explain this phenomenon?
Neurospora, it does not help in this case to present b. Would you find similar patches on the wings of
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the matings in the table as ordered octads. Why not? females having the genotype y sn / y sn ?