Page 186 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

178 Chapter 5 Linkage, Recombination, and the Mapping of Genes on Chromosomes

9 nonparental ditype, and 1 tetratype asci were seen. resulting from this mating, 233 were PD, 11 were
From cross ii, the results were 24 parental ditype, NPD, and 156 were T.
3 nonparental ditype, and 27 tetratype asci. a. What types of spores are in the PD, NPD, and
a. What are the linkage relationships between the T tetrads?
mating type locus and the other two loci? b. Are the his and lys genes linked? How do you know?
b. Although these two crosses were performed in c. Using the simple equation RF = 100 × [NPD +
Neurospora, you cannot use the data given to (1/2)T]/total tetrads, calculate the distance in map
calculate centromere-to-gene distances for any units between the his and lys genes.
of these genes. Why not?
42. Indicate the percentage of tetrads that would have 0, d. If you think about all the kinds of meiotic events
that could occur (refer to Fig. 5.24), you can see
1, 2, 3, or 4 viable spores after Saccharomyces that the calculation you did in part (c) may sub-
cerevisiae a / α diploids of the following genotypes stantially underestimate RF. What kinds of meioses
are sporulated: (NCO, SCO, or DCO) generated each of the tetrad
a. A true-breeding wild-type strain (with no muta- types in this cross?
tions in any gene essential for viability). e. What incorrect assumptions does the simple RF
b. A strain heterozygous for a null (completely inacti- equation you used in part (c) make about the
vating) mutation in a single essential gene. meiotic events producing each type of tetrad? When
For the remaining parts of this problem, consider could these assumptions nevertheless be correct?
crosses between yeast strains of the form a × b, where f. Use your answers to part (d) to determine the
a and b are both temperature-sensitive mutations in number of NCO, SCO, and DCO meioses that
different essential genes. The cross is conducted un- generated the 400 tetrads.
der permissive (low-temperature) conditions. Indicate
the percentage of tetrads that would have 0, 1, 2, 3, or g. Use your answers to part (f) to write a general
4 viable spores subsequently measured under restric- equation that relates the number of DCO meioses
tive (high-temperature) conditions. to the number of the various tetrad types. Then
write another general equation that computes the
c. a and b are unlinked, and both are 0 m.u. from the number of SCO meioses as a function of the num-
centromere. ber of the various tetrad types.
d. a and b are unlinked; a is 0 m.u. from the centro- h. Based on your answer to part (f), calculate the
mere, while b is 10 m.u. from the centromere. average number of crossovers per meiosis (m)
e. a and b are 0 m.u. apart. between his and lys.
f. a and b are 10 m.u. apart. Assume all crossovers i. Use your answer to (h) to write an equation for m
between a and b are SCOs (single crossovers). in terms of NCO, SCO, and DCO meioses.
g. In part (f), if a four-strand DCO (double crossover) j. What is the relationship between RF and m?
occurred between a and b, how many of the spores
in the resulting tetrad would be viable at high k. Use your answer to part (j) to write a corrected
temperature? equation for RF in terms of SCO, DCO, and NCO
meioses.
43. The a, b, and c loci are all on different chromosomes
+
+
in yeast. When a b yeast were crossed to a b yeast l. Using your answer to part (g), rewrite the corrected
and the resultant tetrads analyzed, it was found that RF equation from part (k) in terms of the numbers
the number of nonparental ditype tetrads was equal to of the various tetrad types.
the number of parental ditypes, but there were no tet- m. The equation you just wrote in part (l) is a corrected
ratype asci at all. On the other hand, many tetratype equation for RF that takes into account double
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asci were seen in the tetrads formed after a c was crossovers that would otherwise have been missed.
+
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crossed with a c, and after b c was crossed with Use this improved formula to calculate a more
+
b c. Explain these results. accurate distance between the his and lys genes
44. This problem leads you through the derivation of a than the one you calculated in part (c).
corrected equation for RF in yeast tetrad analysis that 45. a. In ordered tetrad analysis, what is the maximum
takes into account double crossover (DCO) meioses. RF that you could observe between a gene and the
A yeast strain that cannot grow in the absence of centromere? (Hint: What RF would you observe
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the amino acid histidine (his ) is mated with a yeast between a gene and the centromere if the gene
strain that cannot grow in the absence of the amino were distant enough from the centromere so that
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acid lysine (lys ). Among the 400 unordered tetrads at least one crossover, and often more than one