SEGREGATIONS IN ESCHERICHIA COLI 513 



nutritional requirement, i.e., either biotin, methionine, threonine, leucine, or 

 thiamin, allowing the proliferation of the corresponding single mutant as well 

 as the prototrophic type. Colonies were then picked at random and scored ac- 

 cording to their nutritional requirements. The results are summarized in table 

 4. Unfortunately, it was found that the addition of methionine to the minimal 

 medium allowed excessive growth of B~M~, presumably because of a degree of 

 contamination of the methionine with biotin. This datum is, however, not 

 essential for the argument. In general, it will be seen that the + classes are 

 markedly and significantly more frequent than the single mutant types, with 



Table 4 



Relative frequency of various biochemical recombination 

 classes in the cross. 



B-M'T+L+Bi+XB+M+T-L-Br* 



* Cells of the parental types were mixed and plated into agar supplemented with the growth 

 factor indicated. On this medium, the two recombination classes indicated on each line of the 

 table could form colonies. Contrasting alleles only are specified; other loci, unless otherwise speci- 

 fied, have the " + " configuration. The x 2 for the ratio of single biochemically deficient types to 

 prototrophs is calculated for a comparison with the 1 : 1 expectation of a random segregation. 

 As can be seen from the x 2 values, the probability that the deviations are due solely to chance is, 

 in each case, less than .001 . 



the exception of B-r which is nearly ten times as frequent as Bi + . Writing the 

 cross as B-M'T+L+B^XB+M+T-L-Br, these results may be interpreted 

 as follows: 



1. B + M+ T+L+Bi+ more frequent than B~M + . Therefore B and M are 

 linked. 



2. T+L+ B + M+Bi+ more frequent than either T~L+ or T+L~. Therefore T 

 and L are linked. 



3. BrB+M+ T+L+ more frequent than B 1 +B+M+. Therefore B x is linked 

 to B and M, but probably not between them. 



One may therefore map these five loci onto not more than two linkage groups, 

 according to the scheme in fig. 2a. In all that follows, the [B-M] and [T-L] 

 combinations will be regarded as single units, since conclusive information as 

 to their relative order has not been obtained. These data so far do not allow 

 any conclusion to be drawn as to whether the regions B X ~[BM] and [TL] are 

 linked or are independent cf each other, since a recombination between them 

 is a necessary requirement for a detectable type. 



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