SEGREGATIONS IN ESCHERICHIA COLI 515 



supplemented agar is homogeneous with the distribution in prototrophs, so 

 that the segregation of these factors is not influenced by the Bi segregation. 



The data in table 5 show that Lac is inclined not to separate from BM, and 

 is therefore regarded as linked to it, while there is a similar linkage of \\ to 

 TL. Since the recombination of Lac with BM is not influenced by the inter- 

 change between Bi and BM, they are on opposite sides of BM as suggested 

 by map 2b. Finally, a scrutiny of the interaction between the Lac and V segre- 

 gations shows that these are not independent of, each other, particularly be- 

 cause of the rarity of the least frequent class. This suggests, then, that the 

 two linkage groups of fig. 2b be combined to give the map of fig. 2c. (The locus 

 of V& on this map is obtained from additional data.) According to this interpre- 

 tation, the rarity of the least frequent Lac-V combination stems from the fact 

 that a triple-crossover is necessary for its production. In fig. 2d, the cross 

 Y-40X Y-53 is interpreted according to the map, with a table citing the regions 

 in which interchange must take place to yield the given types. 



That the first seven factors to be investigated should fall in the same linkage 

 group leads to the inference that there is only a single chromosome in E. coli. 

 This inference is supported by incomplete analyses of the segregations of 8 

 other markers referred to in table 1. None of these factors has been found to 

 segregate independently of the factors which have already been described as 

 belonging to a single linkage group. The possibility that segregation interac- 

 tions may, in some cases, be based upon an inter-chromosomal type of inter- 

 ference (compare Steinberg and Fraser, 1944), has not been ruled out, 

 however. 



The distances recorded in fig. 2c are derived from the recombination totals 

 in tables 5 and 6. However, the distance between [BM] and [TL] cannot be 

 estimated directly, but only the partition of that distance among the regions 

 BM-Lac, Lac-V u and Vi-TL. The relative frequency of the "triple-inter- 

 change" type can be used to estimate the absolute map distances, if it is as- 

 sumed that there is no interference. This frequency, about 2.1 percent, is 

 readily calculated to be consistent with a map length of between 75 and 80 

 units altogether either in a two-strand or a four-strand system (Lederberg, 

 1947). These values must be regarded as rough approximations, because they 

 are extremely sensitive to error in the estimation of the proportion of the 

 "triple" types. 



Linearity 



In constructing a map, and calculating distances, it has been taken for 

 granted that there is in E. coli a system of linear linkage, such as has been 

 demonstrated quite conclusively in Drosophila, and inferred in all higher or- 

 ganisms. What direct evidence may one bring to bear on this question? 



The method which one is forced to em\ loy in hybridizing this bacterium 

 introduces certain complications. The classical proof of linearity is based on 

 the additive character of distances, expressed in morgans, between loci occur- 

 ring within the same linkage group. The determination of map distances is 

 based upon a comparison between parental and new combinations of linked 



'53 



