7. You do a three-factor cross using P1 transduction to determine the relative o

Question

7. You do a three-factor cross using P1 transduction to determine the relative order of three genes on the E. coli chromosome. The donor and recipient phenotypes are as follows: Donor Ara, Rbs', Mal Recipient: Ara, Rbs, Mal After infecting the donor and using the lysate to infect the recipient, you select for Ara* cells by plating on media containing arabinose as the only carbon source. You then screen for the other two markers and get the following number of colonies: Ara Rbs Mal 46 Ara Rbs Mal5'1 Ara Rbs Mal 2 Ara Rbs Mal 42 B. Assume the average length of the transdusing DNA (L) is 1.5 minutes and calculate the map distance (d) between the loci. (4 points) c. Construct a linkage map showing the order of the genes and map distances between them. (6 points)

Explanation / Answer

A. The co-transduction frequency = 2 + 42 /141
                                 = 44/141
                                 = 0.31 = 31%

B. Cotransformation frequency = (1 – d/L)3
        0.31 = (1 – d/1.5)3
        cubic root of 0.31 = (1 – d/1.5)
        0.67 = (1 – d/1.5)
        d/1.5 = 1 - 0.67
        d = 1.5 X 0.33
        d = 0.49 minute

C. To determine linkage map the rare class has to check because it requires two pairs of crossover events, with the middle marker being inherited from the recipient and the two outside markers inherited from the donor.

Here rare class is Ara+ Rbs+ Mal+ = 2. For two cross over has to occur, the order is Ara+ Mal+ Rbs+

Ara+            Mal+    Rbs+
    -----51%----
    ------------2%-------

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