1. A cross is made between an Hfr that is met + thi + pur + and an F - that is m

Question

1. A cross is made between an Hfr that is met+ thi+ pur+ and an F- that is met- thi- pur-. Interrupted mating studies show that the met+ enters the recipient last, and so met+ recombinants are selected on a medium. The recombinants are tested for the presence of thi+and pur+ alleles. The following numbers of individuals are found for each genotype:

            met+ thi+ pur+                        280

            met+ thi+ pur-                        0

            met+ thi- pur+                                                 6

            met+ thi- pur-                         52

a. What media are used to detect the four genotypes above? (Use the notation MM for minimal medium.) (10 points (2.5 each))

Genotype

Medium

met+ thi+ pur+

met+ thi+ pur-

met+ thi- pur+                  

met+ thi- pur-

b. What are the gene order and map distances for these three genes? (15 points.)

Genotype

Medium

met+ thi+ pur+

met+ thi+ pur-

met+ thi- pur+                  

met+ thi- pur-

Explanation / Answer

1) a)

b) Since met enters the recipient last, there are only two possible gene orders if the first marker enters on the right: met, thi, pur or met, pur, thi. One of the four possible classes of recombinants requires two additional crossovers. Each possible order predicts a different class that arises by four crossovers rather than two. For instance, if the order were met, thi, pur, then met+thipur+ recombinants would be very rare. On the other hand, if the order were met, pur, thi, then the four-crossover class would be met+purthi+. From the information given in the table, it is clear that the met+purthi+ class is the four-crossover class and therefore that the gene order is met, pur, thi.

Distance between met and pur is

= 52/338

= 15.4 m.u.

Distance between pur and thi is

= 6/338

= 1.8 m.u

Gene map:

met pur thi

I---------------------------------------------------I----------------I

15.4 m.u 1.8 m.u

Genotype medium met+  thi+ pur+ MM supplemented with met+ thi+ pur+ met+ thi+ pur- MM supplemented with met+ thi+ met+  thi- pur+ MM supplemented with met+ pur+ met+  thi- pur- MM supplemented with met+

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