Vol. 35, 1949 GENETICS: J. LEDERBERG 179 



abnormalities in the segregation ratios suggest some sort of chromosomal 

 aberration whose nature has not yet been proved. 



Material and Methods. — The cultural and crossing techniques used earlier 

 have generally been followed. Crosses were also made by spreading 

 mixtures of cells on the surface of synthetic medium 2 to which carbo- 

 hydrate and an eosin-methylene blue indicator are added. This medium, 

 EMS-, classifies prototrophs as soon as they appear for fermentation of the 

 sugar used. Other media used include EMB- agar, in which the indicator 

 is incorporated in a sugar peptone base. The genetic factors involved in 

 this study are summarized in table 1. 



"Het" . . . . Persistent heterozygote factor 



Crosses Involving "Het." — -The first unstable prototroph to be discovered. 

 "H-l," arose in a cross to test the allelism of the virus resistance factors 

 Fi and V u . The parents were B - M - Lac + IV and T - L - Bi - Lac - V*. 

 Among 200 prototrophs picked and tested on synthetic agar for sensitivity 

 to Tl, only one was an apparent crossover showing a full sensitive reaction. 

 However, when this unique culture was streaked out on EMB— lactose, it 

 gave rise not to typical dark Lac+ or light Lac— colonies, but to highly 

 variegated colonies with intermingled sectors of light and dark cells. The 

 variegated reaction will be referred to as Lacy. 



When Law colonies were streaked out again on EMB — lactose, pure 

 Lac — , pure Lac+ and a few Lacy colonies were seen. But when the 

 pure — and + colonies were inoculated on EMS — lactose they failed to 

 grow, showing that they were nutritionally deficient. Furthermore, 

 although they had been derived from a virus-sensitive culture, most of the 

 purified "segregants" were resistant to Tl when tested on EMB— lactose. 

 On the other hand, Lacy colonies gave rise to normal appearing Lac-f- on 



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