516 JOSHUA LEDERBERG 



genes, as determined in the progeny of zygotes selected at random. In E. coli, 

 on the other hand, one is limited to the recovery of that recombination class 

 in which there has necessarily been an interchange between certain biochemical 

 loci, in the cases here discussed, betwen [BM] and [TL]. For this reason, it is 

 not possible to obtain a direct measure of the absolute distance between factors 

 which are located within this critical region, and any argument in favor of 

 linearity which is based on the segregations of such factors may have the 



a.— + 



A k A [J" L] 



b.-H=-nt 



B, \tf £} L.c if [r 



Crossovers Recoverable type 



(c,d, ore) B l -M+B+T+L+ 



(a) (c, d, or e) B 1 - r M+B+T+L+ 



(a) (b) B 1 +M+B-T+L+ 



(a) (/) B 1 +M+B+T-L+ 



(a) (/) (c, d, or e) B.+M+B+T+L- 



(a) (c) B 1 +M+B+T+L+ ■ ■ ■ Lac+Vf 



(a) (d) • • • Lac-Vf 



• • • Lac-Vf 



(a) (c) (d) (e) ••• Lac + \\> 



Figure 2. — a, b, and c. Mapping of genetic factors, d. The cross B{* M~B~ Lac' h V \ r T + L + XB{ 

 M^B + Lac~Vi'T~L~ and some of the recoverable crossover classes. (See table 5.) 



flavor of circular reasoning. It would be preferable to study the segregations of 

 factors which are assigned to loci distal to the biochemical factors whose re- 

 combination is the basis of the detection of sexual offspring. The stocks with 

 which this might be accomplished are not yet available, but it is hoped that 

 they will be for future work. 



That there does exist some sort of linkage system is made highly credible by 

 the results of the "reverse crosses" tabulated in tables 3 and 5. The chief diffi- 

 culty in proving that this system is linear has been to formulate the feasible 

 alternatives, so that critical experiments, the results of which could discrimi- 

 nate between linearity and a given alternative, might be set up. Certain types 

 of "linkage" can be disqualified by the data already at hand. For example, one 

 might postulate that genes of bacteria are embedded in a two-dimensional 



154 



