An Hfr strain of genotype: arg- lac- leu+ strs is mated with a recipient strain of genotype: arg+ lac+ leu- strr. The strs marker is known to be transferred very late. Among the other three markers, leu+ is the last one to enter the F- cell. (Note: All bacteria have the ability to utilize glucose as a sugar source. The alleles arg+and leu+ are necessary for synthesis of arginine and leucine respectively. The lac+ is necessary for utilization of lactose.)


Exconjugants are plated on a medium containing glucose, streptomycin, and arginine. The following results are obtained for 500 colonies isolated from these plates and tested for growth on various media:


Minimal medium (with glucose): 135 colonies

Minimal medium (without glucose) + lactose: 85 colonies

Minimal medium (without glucose) + lactose + arginine: 90 colonies


a. Write a list of possible genotypes from those 500 colonies, and indicate the number in each genotype. Show your work and circle your answer.

b. Calculate the recombination frequencies.