Phage Cross: Infect bacteria with 10 of each phage per cell to insure that every cell has both kinds of phage replicating inside. Allow one round of growth then plate out the morphology of the plaques Plaque type Frequency lg clear (parent 0.4 sm turbid (paren 04 lg turbid (recomb. 0.1 sm clear (recomb.) 0.1 For this cross the measured distance between mi and cr is number of recombinants 100×100 0 m u(map units) We can also use this system to find the distance between two mutations in the same gene. For example, lets say that we have isolated two different cr mutants that give clear plaques. After infection of E. coli with w10 phage per cell of each mutant, the resulting phage are plated and their plaque morphology is examined. Most of the phage will still be ef, but recombination between mutations will produce wild type phage with turbid plaques. If 2 out of 1,000 phage make turbid plaques then the measured map distance between the two mutations is 0. 4 m u. Note that in this example we are only counting wild type recombinants and there should be an equal number of double-mutant recombinants- both classes of recombinants must be accounted for in calculating the map distance -1 cI-2Phage Cross: Infect bacteria with ~10 of each phage per cell to insure that every cell has both kinds of phage replicating inside. Allow one round of growth then plate out the phage that are produced to look at the morphology of the plaques. Plaque type Frequency lg clear (parent) 0.4 sm turbid (parent) 0.4 lg turbid (recomb.) 0.1 sm clear (recomb.) 0.1 For this cross the measured distance between mi– and cI– is: number of recombinants 20 x 100 = x 100 = 20 m.u. (map units) total 100 mi cI 20 mu We can also use this system to find the distance between two mutations in the same gene. For example, let’s say that we have isolated two different cI– mutants that give clear plaques. After infection of E. coli with ~10 phage per cell of each mutant, the resulting phage are plated and their plaque morphology is examined. Most of the phage will still be cI–, but recombination between mutations will produce wild type phage with turbid plaques. If 2 out of 1,000 phage make turbid plaques then the measured map distance between the two mutations is 0.4 m.u. Note that in this example we are only counting wild type recombinants and there should be an equal number of double-mutant recombinants — both classes of recombinants must be accounted for in calculating the map distance. cI-1 cI-2 0.4 mu