SEGREGATIONS IN ESCHERICHIA COLI 511 



For most purposes, however, this contamination may be ignored, as will be 

 shown in a later section. Prototroph colonies are then fished and streaked di- 

 rectly on EMB plates, or otherwise tested, to classify them with respect to 

 other factors that may be segregating. 



RESULTS AND CONCLUSIONS 



In most organisms inheritance is studied by the examination of zygotes 

 carrying the gene alternatives determining a character. The segregants are 

 chosen at random, and factor linkage is recognized by deviations in the fre- 

 quency of parental and new couplings of a series of characters. In the absence 

 of a random method of separating zygotes in E. coli, one is limited here to the 

 members of specific recombination classes, namely the prototrophs. It is how- 

 ever, possible to introduce other factor differences into the biochemical mu- 

 tants from which prototrophs are obtained, and to determine how such factors 

 segregate into this recombination class. It was hoped in this way to obtain in- 

 formation concerning the haploid or diploid condition of the bacterial cell, and 

 to determine whether factors segregated at random, or according to specific, 

 perhaps linear chromosomal laws. 



The first factor pair to which this approach was applied was Vi r /V\* (Leder- 

 berg and Tatum, 1946b). In the cross B-M-P+T+VSXB+M+P-T-V^, ten 

 B+M+P+T+ were isolated. Eight proved to be V{ while two were V x \ This at 

 once suggested that the vegetative cell of E. coli is haploid, since segregation 

 could be observed in the first filial generation clone. It was noted also at that 

 time that the "reversed" cross: B-M-P + T+V{XB+M + P-T-Vi T gave quite a 

 different ratio of r/s in the prototrophs, namely 3:7. Results on so small a 

 sample are of doubtful significance, but they suggested the technique by which 

 the basis of this character "segregation" could be elucidated. For this reason, 

 the study of "reversed" crosses was extended to include numerically more 

 data, using various combinations of mutants, and involving in addition to 

 V! r /Vi\Lac + /Lac~. The information which was obtained is summarized in 

 tables 3 and 5. The data show clearly that neither of the factor alternatives 

 V\ r /V\* or Lac + /Lac~ segregates at random into the prototroph recombination 

 class. However, the occurrence of all factor combinations, albeit with different 

 frequencies, is evident, at least with respect to Lac and V\. It seemed clear 

 that there are only two alternative explanations for the unequal frequencies 

 with which alternative alleles are manifested in the prototrophs: (a) that the 

 alleles were characterized by some differential physiological property, such as 

 dominance, or preferential segregation, or (b) that the nonrandom segregation 

 was due purely to the mechanics of factor recombination, which is to say a 

 linkage system. 



The results of "reversed crosses" have a distinct bearing on this problem. If 

 nonrandom segregation into protctrophs were due to some physiological prop- 

 erty of the allele concerned, its particular coupling in the parent in which it is 

 introduced should have no great effect on the segregation frequency; if on the 

 other hand, the effect were purely mechanical, the segregation would reflect 

 entirely the couplings of the parents, and the substitution of one allele for 



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