A cross is made between Hfr met+ thi+ pur+ and F- met- thi- pur-. Interrupted ma

Question

A cross is made between Hfr met+ thi+ pur+ and F- met- thi- pur-. Interrupted mating studies show that met+ enters the recipient last, so met+ ex-conjugant (recipient) cells are selected on medium containing thi and pur only. These ex-conjugants 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. Why are there no individuals of the met+ thi+ pur- genotype?

Explanation / Answer

(a) to allow selection for met+ recombinants (last marker to enter); ensures that all the loci considered in the cross will have already entered each recombinant that we analyze

(b) met pur thi

(c) met-pur 15.4 mu; pur-thi 1.8 mu

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