Chapter 7, Problem 35P

In a haploid yeast strain, eight recessive mutations were found that resulted in a requirement for the amino acid lysine. All the mutations were found to revert at a frequency of about $1\times {10}^{-6}$ except mutations 5 and 6, which did not revert. Matings were made between a and α cells carrying these mutations. The ability of the resultant diploid strains to grow on minimal medium in the absence of lysine is shown in the following chart (+ means growth and − means no growth.)

 

a.	How many complementation groups were revealed by these data? Which point mutations are found within which complementation groups?
	The same diploid strains are now induced to undergo sporulation. The vast majority of resultant spores are auxotrophic; that is, they cannot form colonies when plated on minimal medium (without lysine). However, particular diploids can produce rare spores that do form colonies when plated on minimal medium (prototrophic spores). The following table shows whether (+) or not (−) any prototrophic spores are formed upon sporulation of the various diploid cells.
b.	When prototrophic spores occur during sporulation of the diploids just discussed, what ratio of auxotrophic to prototrophic spores would you generally expect to see in any tetrad containing such a prototrophic spore? Explain the ratio you expect.
c.	Using the data from all parts of this question, draw the best map of the eight lysine auxotrophic mutations under study. Show the extent of any deletions involved, and indicate the boundaries of the various complementation groups.