In an E. coli mapping experiment a cross is made between an Hfr strain that is a

Question

In an E. coli mapping experiment a cross is made between an Hfr strain that is a+ b+ d+ in genotype and an F- strain that is a- b- d- in genotype. Interrupted-mating studies show that b+ enters the recipient strain last. You select for b+ recombinants and obtain 400 that are then tested for the presence of the a+ and d+ alleles. The following data were obtained:

                     a+ b+ d+     326

                     a-   b+ d+        2

                     a+ b+ d-       14

                     a-   b+ d-       58

                                        400

A. (2 points) why select for the late entering gene in this experiment?

B. (2 points) What is the gene order (show your reasoning)?

C. (4 points) What are the frequencies of recombination between each pair of genes? Show calculations.

D. (2 points) Draw a map, showing map distances (in map units) between genes.

Explanation / Answer

A) In this experiment, b+ enters the recipient strain last. The recipient locus is situated very far away from b+. So, it never enters the recipient strain again. Therefore, the b+ recombinants are then tested for the presence of the a+ and d+ alleles.

B) The double recombinant is a-b+d+. So, the middle allele is a. The double crossing over event moves the middle allele over other alleles. Therefore, the gene order is 'd a b'.

C) Recombination frequency (RF) is calculated by the formula,

RF = total recombinants/total offsprings x 100

= 74/400 x 100

= 18.5%

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