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In a generalized transduction system using P1 phage, the donor is pur+ nad+ pdx- and the recipient is pur- nad- pdx+. The donor allele pur+ is initially selected after transduction, and 50 pur+ transductants are then scored for the other alleles present. The results follow:
Genotype # of colonies
nad+ pdx+ 3
nad+ pdx- 10
nad- pdx+ 24
nad- pdx- 13
Total 50

a. What is the cotransduction frequency for pur and nad?
b. What is the cotransduction frequency for pur and pdx?
c. Which of the unselected loci is closest to pur?
d. Are nad and pdx on the same side or on opposite sides of pur? Explain. (Draw the exchanges needed to produce the various transformant classes under either order to see which requires the minimum number to produce the results obtained)