Two-factor Crosses

Phage P22 HT was grown on the donor cells shown below. Using these phage lysates, two-factor crosses were done against a strain of Salmonella with mutations in the metA, aceA, and iclR genes. Given the results shown in the table below, draw a genetic linkage map showing the cotransduction frequency and the predicted order of the metA, aceA, and iclR genes. (Note that the mutation in the metA gene is due to a Tn10 insertion which encodes tetracycline resistance.)

DonorRecipient cellsSelected phenotypeRecombinantsNumber obtained
metA::Tn10 iclR-metA+ iclR+ TetRIclR-
aceA+ iclR- aceA- iclR+Ace+ IclR-
metA::Tn10 aceA+ metA+ aceA- TetRAce-

ANSWER: Note that you need to determine the frequency of coinheritance -- that is, the fraction of recombinants that inherited both DONOR alleles. First determine the number of recombinants obtained which inherited both donor alleles (pay attention to the donor genotype because the donor alleles are not always listed first in data tables as shown in the third calculation below). Divide this number by the total number of recombinants obtained to determine the fraction of recombinants that co-inherited both donor alleles. The calculations are shown below.

Given the calculated co-inheritance frequencies, predict the relative locations of the mutations based upon the logic that the closer together two alleles are in the donor, the more likely they will be co-inherited (and vice-versa). A best fit for the predicted genetic map is shown below. (It is important to include arrows to indicate the selected marker because sometimes reciprocal crosses give different co-inheritance frequencies.)