Sexual reproduction occurs in the bread mould (Neurospora crassa) when two haploid strains of different mating types come into contact with each other. The haploid cells fuse to form a diploid cell, which then undergoes meiosis to produce haploid spores. The spores are dispersed and grow into new fungal colonies as haploid individuals.

In Neurospora, two genes participate in the synthesis of valine. The wild-type allele for one gene is designated val-1+ and a mutant allele designated val-1-. The second gene has a wild-type val-2+ allele and a mutant val-2- allele. These two genes are linked, with a map distance between them of 25%.

a. Progeny from the cross: val-1- val-2+ X val-1+ val-2- are plated on minimal medium. What proportion of the progeny will grow on minimal medium?

b. The val-1- val-2+ strains accumulate intermediate compound B, and the val-1+ val-2- strain accumulate intermediate compound A. Draw the two possible linear pathways that will explain these results. Include on your pathway each of the chemicals mentioned in the question and valine. Also indicate which gene product (val1+ product or val2+ product) controls which step.

c. The val-1- val-2+ strains will grow if the minimal medium is supplemented with valine or compound A, however the val-1+ val-2- strain will grow only on medium supplemented with valine and not with compound B. Draw a table to describe these results, indicating the growth abilities of each strain on:

Strain Minimal medium (MM) MM + val MM + A MM + B

d. Indicate which of your pathways in part b is the correct one.

e. Which compound will be accumulated by the double mutant val-1- val-2- ?