Chapter 5, Problem 46P

A research group has selected three independent Trp− haploid strains of Neurospora, each of which cannot grow in the absence of the amino acid tryptophan. They first mated these three strains with a wild-type strain of opposite mating type, and then they analyzed the resultant octads. For all three matings, two of the four spore pairs in every octad could grow on minimal medium (that is, in the absence of tryptophan), while the other two spore pairs were unable to grow on this minimal medium.

a.	What can you conclude from this result? In the matings of mutant strains 1 and 2 with wild type, one of the two topmost pairs in some octads had spores that could grow on minimal medium while the other of the two topmost pairs in the same octads had spores that could not grow on minimal medium. In the mating of mutant strain 3 with wild type, either all the spores in the two topmost pairs could grow on minimal medium or all could not grow on minimal medium.
b.	What can you conclude from this result? The researchers next prepared two separate cultures of 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 octads, and determined how many of the individual spores could grow on minimal medium. The results are shown here. Mating % of octads with x number of spores viable on minimal medium $x=0$ 2 4 6 8 $1\times 2$ 78 22 0 0 0 $1\times 3$ 46 6 48 0 0 $2\times 3$ 42 16 42 0 0
c.	For each of the three matings in the table, how many of the 100 octads are PD? NPD? T?
d.	Draw a genetic map explaining all of the preceding data. Assume that the sample sizes are sufficiently small that none of the octads are the result of double crossovers.
e.	Although this problem describes crosses in Neurospora, it does not help in this case to present the matings in the table as ordered octads. Why not?
f.	Why in this problem can you obtain gene ↔ centromere distances from the crosses in the table, even though the data are not presented as ordered octads?