Name 2. a. In E. coli, four Hf strains donate the following markers, shown in th
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Question
Name 2. a. In E. coli, four Hf strains donate the following markers, shown in the o donated: strain 1 Q B X T A strain 2 G CY S A strain 3 X B Q D G strain 4 S A T X B All these Hfr strains are derived from the same P+ strain. What is the order of these markers on the circular chromosome of the original F+? (10 pts.) b. In an Hfr X F-cross, leu+ enters as the first marker, but the order of the other markers is unknown. If the Hir is wild-type, and the F- is auxotrophic for each marker in question, what is the order of the markers in a cross where leu+ recombinants are selected if 27% are i let, 13% are mal+. 82% are the+, and 190 are trp+2 (5 pts.) c. A cross is made between a different (from a different Ft- may or may not have the same gene order) Hfr that is C+ G+ D+ and an F- that is C- G- D-. Interrupted mating studies show that C+enters the recipient last, so that C+ recombinants are selected on a medium containing the supplements that satisfy only the G and D requirements. These recombinants were tested for the presence of the G+ and D+ alleles. The following number of individuals are found for each genotype: C+ G+ D+ 280 C+ G D 0 C+ G- D+ 6 C+ G- D- 52 What is the gene order and state your reasoning? (5 pts.) d. What is the map distance in map units between C and G? (10 pts.)Explanation / Answer
b.
A recipient cannot receive all the markers due to spontaneous breakage of the genetic material. Therefore, a natural gradient of transfer is created which reflects in the frequency of recombinants, as a function of the order of entry for each marker. Therefore, the order markers can be identified by the percentage of recombinants. The more is the percent of recombinants for a particular marker, the more nearer it is in the order of genes.
Since the recombination frequency of Thr+ is the highest, it must be the first marker to enter after leucine. Likewise, the order can be deduced as leu, thr, ile, mal, and trp.
c.
We know that C+ enters the recipient last, so the only two possible markers to enter first are either G or D. Therefore, the order must be either G+ D+ C+ or D+ G+ C+. If the order is D G C, then D+ G- C+ recombinants would be very rare.
On the other hand, if the order were G D C, then the class G+ D- C+ would be the rare class. The results show that the number of progeny with G+ D- C+ class are 0.
Therefore, the order of the genes is G, D, and C.
The recombination frequency between G and D is 52/388 = 15.4 mu.
The recombination frequency between D and C is 6/338 = 1.8 mu
d.
Therefore, the distance between C and G is 17.2 mu.
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