there is evidence that other loci, too, must take part 

 in building full resistance. These data are therefore 

 consistent with the hypothesis of many loci with 

 cumulative action. 



First-step resistants, obtained by both plate and 

 test-tube selection methods, were analysed in crosses 

 of sensitive with resistant and resistant with resistant, 

 and they showed a clear-cut segregation of sensitivity 

 or resistance, within experimental error. Mapping 

 loci of first-step resistance, the loc;:s most frequently 

 met so far was closely linked to F,. In this way, a 

 cross with four free markers between BM and TL 

 (fixed recombination markers) was made, involving 

 Chloromycetin resistance, V t , Lac and V lt giving a 

 relatively satisfactory test of linearity of the chromo- 

 some region between BM and TL. First -step resist- 

 ants were also found which gave no resistant recom- 

 binants when crossed to sensitive, probably because 

 of close linkage with fixed recombination markers 

 (BM in this case). The interactions in resistance 

 between different loci are under investigation at 

 present. In fact, this case seems to offer un- 

 usual possibilities of a detailed analysis of the inter- 

 actions between genes in quantitative inheritance. 

 The stability of the drug is a factor of no little 

 importance in facilitating quantitative work with 

 Chloromycetin. 



The K-12 strains used in this research have been 

 kindly sent by Dr. J. Lederberg. One of us (G. A. M.) 

 has been supported during this work by a grant from 

 the Rockefeller Foundation. 



L. L. Cavalli 

 G. A. Macoaoaro 



Department of Genetics, 



Whittingehame Lodge, 



44 Storey's Way, 



Cambridge. 



Aug. 15. 



1 Demerec, M., Ann. Mist. Hot. Oard., 82, 131 (1946). 



• Lederberg, J., and Tatum, E. L., Nature, 168, 558 (1946). 



1 Newcombe, H. B., and Hawirko, R., J. Bad., 67, 565 (1949). 

 4 Demerec, M., Amer. Nat., 84, 5 (1950). 

 ' Newcombe, H. B. (unpublished). 



• Lederberg, J., J. Bad., 69, 211 (1950). 

 ' Lederberg J., Genetics, 32, 505 (1947). 



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