A three-factor transduction study of the a , d , and e loci is carried out. Bact
ID: 170312 • Letter: A
Question
A three-factor transduction study of the a, d, and e loci is carried out. Bacteria of the genotype a- d- e- are infected with transducing particles obtained following phage infection of bacteria with the genotype a+ d+ e+. Using appropriate selection techniques, cells transduced for the a locus are identified and then tested further to determine how many of them have been transduced for one or both of the other loci. The genotypes of the transductants and their frequencies are as follows:
Genotype
Frequency
a+d+e+
160
a+d+e-
10
a+d-e+
120
a+d-e-
820
A. Calculate the cotransduction frequency for a and d. (2 points)
B. Calculate the cotransduction frequency for a and e. (2 points)
C. Assume the average length of the transducing DNA (L) is 2.5 minutes and calculate the map distance (d) between the loci. (4 points)
D. Construct a linkage map showing the order of the genes and the distances between all loci. (6 points)
Genotype
Frequency
a+d+e+
160
a+d+e-
10
a+d-e+
120
a+d-e-
820
Explanation / Answer
The cotransduction frequency is calculated by the ratio of transductants that has coinherited both markers, divided by the total number of transductants.
You can use the Wu formula to estimate the correlation between cotransduction frequency and the physical distance between two genetic markers.
C = 1 - [d/L]3
C = cotransduction frequency; L = size of transducing fragment (kB)
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