SUMMARY: When colicinogenic female bacteria are crossed with non-colicinogenic males, all the genes of the male chromosome can enter into the zygote without necessarily killing it. There is a great difference with regard to the number of certain recombinants tested, derived from crosses of the type Hfr col x Fcol, as compared to their numbers in crosses of the type Hfr col x Fcol. In certain cases this difference was reduced when the zygotes were incubated at 28° instead of the standard temperature of 37°. It was found that resistance to colicines is not always an all-or-none phenomenon. There are different degrees of colicine resistance and those mutants which show complete resistance by the standard tests are still found to be sensitive under more rigorous conditions. In the case of colicine B the sensitivity of these semi-resistant mutants is greater at 37° than at 28°.


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