182 GENETICS: J. LEDERBERG Proc. N. A. S. 



region including this locus. This tentative conclusion is in accord with 

 the aberrant segregation of Lac (excess of Lac — ) observed in H-l. The 

 hypothetical deficiency would be lethal in a haploid, and lead to the loss of 

 segregants carrying the alleles linked to it. 



Some quantitative studies of segregation and crossing over have been 

 carried out on these complex heterozygotes. H-72 segregates both for 

 lactose and xylose fermentation. Segregation was permitted to occur 

 "enmasse" in heavy cultures in nutrient broth, the cultures diluted and 

 plated on EMB- xylose and EMB -lactose. Of 895 colonies on EMB- 

 xylose, 19 were still Xyto; 815 Xyl- and 61 Xyl+. The Xyl+ were 

 all Lac—. The segregation ratio is 815— : 61+ or more than 13:1. 

 Similarly, of 753 colonies on EMB— lactose, 23 were still Lacf; 654 Lac — 

 and 76 Lac-}- (all of which were Xyl — ). Thus, no Lac-f-Xyl-|- segre- 

 gants were observed in this sample. The other classes are computed to 

 be Lac-Xyl- 83%, Lac-Xyl+ 7% and Lac+Xyl- 10%. These 

 figures might be accounted for by mapping : 



Xyl+ 7.7 Lethal 11.2 Lac+/Xyl- . . .Lac-. 



To justify these conclusions, independent mapping of Xyl, Mai, Lac 

 will be necessary. 



Four hundred ten segregants were tested both on EMB— lactose and on 

 EMB — xylose. Segregation was always complete; i.e., no cultures were 

 found which had segregated for one factor, and not for the other. This 

 eliminates interpretations based on new segmental losses, or on some forms 

 of autogamy. 



It may be argued that the unequal segregations are due to selection. 

 This factor cannot be excluded, but the dominant segregant differs from 

 diploid to diploid. That is to say, other heterozygotes have been isolated 

 from parallel crosses which gave an excess of Lac+ rather than Lac — 

 segregants, or Xyl+ more than Xyl— or both. It seems more likely that 

 the ratios observed are based upon the genetic constitution of the hetero- 

 zygote. 



The Lac-}- character of Laci heterozygotes shows that Laci+ is domi- 

 nant to Laci — . A number of other loci have been identified, mutations 

 at which lead to the inability to ferment lactose. At two more of these 

 at least, the + allele is dominant. Cultures carrying Lac 2 — or Lac 4 — 

 were crossed to "Het" stocks carrying Laci — . A large proportion of the 

 Lac+ prototrophs seen on EMS — lactose turned out to segregate for lactose 

 fermentation. Their variegation, however, was generally periclinal 

 rather than sectorial, with dark, Lac-|- centers and light, Lac — margins 

 with only occasional streaks of Lac-|-. This difference is to be expected, 

 since each of the chromosomes of the diploid carries a Lac— mutation. 

 Therefore, only those segregants in which there has been a crossover 



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