) An Hfr strain (arg+, bio+, leu+) is mated to an F- strain (arg-, bio-, leu-).
ID: 209347 • Letter: #
Question
) An Hfr strain (arg+, bio+, leu+) is mated to an F- strain (arg-, bio-, leu-). Assume that this Hfr strain transfers arg+ last. You grow the exconjugants on arg- media, supplemented with leu+ and bio+. You recover 423 arg+ exconjugants. These are tested individually for bio and leu and you find the following distribution of genotypes:
arg+, bio+, leu+ 360
arg+, bio+, leu- 9
arg+, bio-, leu+
0 arg+, bio-, leu- 54
a) What is the gene order (show your reasoning)? b) What are the frequencies of recombination between each pair of genes?
Explanation / Answer
exconjugants were grown on leu+ and bio+ media. arg+ enters last, for that reason arg+ marker has been avoided in the media. so that selection for arg+ gets carried out. If arg+ enters last, the two remaining combinations are arg bio leu or arg leu bio. Out of the four possible recombinants, two should be rare. Because they require additional crossovers. If the order was arg bio leu, then it becomes arg+ bio- leu+, which becomes rare. If the order was arg leu bio, then it becomes arg+ leu- bio+, then it becomes rare. already it was found that arg+, bio-, leu+ ware rare, so the order is arg bio leu. The recombinant frequencies are........... total number of recombinants,,,,,,,,360+9+54 = 423 recombinant frequency between arg bio is...........54/423 x100 =12.76 m.u. recombinant frequency between bio leu is...............9/423 x100= 2.12 m.u.
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