178 Chapter 5 • The Genetics of Bacteria and Their Viruses 3. In an Hfr 3 F 2 cross, leu 1 enters as the ±rst marker, but the order of the other markers is unknown. If the Hfr is wild-type and the F 2 is auxotrophic for each marker in question, what is the order of the markers in a cross where leu 1 recombinants are selected if 27 percent are ile 1 , 13 percent are mal 1 , 82 percent are thr 1 , and 1 percent are trp 1 ? Solution Recall that spontaneous breakage creates a natural gradi-ent of transfer, which makes it less and less likely for a re-cipient to receive later and later markers. Because we have selected for the earliest marker in this cross, the fre-quency of recombinants is a function of the order of entry for each marker. Therefore, we can immediately deter-mine the order of the genetic markers simply by looking at the percentage of recombinants for any marker among the leu 1 recombinants. Because the inheritance of thr 1 is the highest, this must be the ±rst marker to enter after leu. The complete order is leu, thr, ile, mal, trp. 4. A cross is made between an Hfr that is met 1 thi 1 pur 1 and an F 2 that is met 2 thi 2 pur 2 . Interrupted-mating studies show that met 1 enters the recipient last, so met 1
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.