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Bacterial and viral recombination

A cross is made between Hfr met+ thi+ pur+ X F- met- thi- pur-.

Interrupted-mating studies show that met+ enters the recipient last, so met+ exconjugants are selected on medium containing thi and pur only. These exconjugants are tested for the presence of thi+ and pur+. The following numbers of individuals are found with each genotype:

met+ thi+ pur+ 280
met+ thi+ pur- 0
met+ thi- pur+ 6
met+ thi- pur- 52

(a) Why was met left out of the selection medium?
(b) What is the gene order?
(c) What are the map distances in recombination units?