362 



CELL HEREDITY 



Brown pigment 



cd + 



3-OH-Kynurenme 



Kynurenme 



1 



Formylkynurenine 



I, 



w+ 



Tryptophane 



FIGURE 12.4. The biochemical basis of the genetic control of eye color in Drosophila 

 (from Wagner and Mitchell, 1955, Genetics and Mefabolism, New York, Wiley; after 

 Beadle, 1945, Chem. Rev., 37:15). 



established this and other substances in sequence as precursors of the 

 brown pigment in the insect eye (Figure 12.4). 



The reparable versus irreparable nature of lethals has also been investi- 

 gated in Drosophila with the use of somatic crossing over with the same 

 y and sn genes shown in Figure 12.2. If, for example, there were a 

 lethal gene adjacent to the yellow marker on the upper chromosome, and 

 if twin spots regularly occurred, it would be concluded that the lethal 

 does not prevent the survival of a patch of cells homozygous for it as 

 long as the rest of the animal remains heterozygous. On the other hand, 

 if single singed spots were of regular occurrence, we would know they 

 were the survivors of segregations that would have led to twin spots ex- 

 cept that the yellow-lethal homozygote did not survive. Demerec found 

 with the use of this method that there are both kinds of sex-linked lethals. 

 Most lethals that are demonstrated to be deficiencies are also autonomous 

 and are cell lethals, but those that survive when present on the single 

 X of the male make for viable cells. Some deficiences are so restricted 

 in scope as to not include an irreparable functional unit that even haploid 

 "organisms " such as phages survive their presence. 



Somatic crossing over is not restricted to the X chromosome of Dro- 

 sophila. It occurs between other chromosomes and in other organisms, 

 such as yeast and fungi. An extensive analysis of the process has been 

 carried out by Pontecorvo and his associates in Glasgow with the fungus, 

 Aspergillus nidulans, where the products of somatic crossing over can be 

 recovered and analyzed. This organism contains haploid nuclei and pro- 



