410 GENETICS OF SOMATIC CELLS 



caution, because of the many unknowns interposed by the developmental and 

 organizational processes in cells of higher organisms. 



STUDIES ON SOMATIC VARIATION THROUGH PHENOTYPIC MARKERS 



Because of differences in methodology, in the nature of the problems involved, 

 and in the type of information now available, this chapter will be subdivided according 

 to the biological materials into a consideration of pertinent studies on (1) normal somatic 

 cells in vivo, (2) neoplastic cells in vivo, and (3) tissue-culture work. Since the subject 

 matter that might be considered is overwhelmingly large, attention will be mainly 

 focused on certain aspects that have caught the interest of the author. No claim of 

 completeness can be made ; other reviewers would have emphasized and discussed other 

 aspects of this huge field where so little is known and so much may be pertinent. Ani- 

 mal, particularly mammalian, material will be considered most and information on 

 other groups will only be touched upon when appearing particularly relevant. 



NORMAL SOMATIC CELLS in vivo 



Somatic crossing over. — This mechanism will be considered separately, because, as 

 pointed out below, it may permit the genetic analysis of somatic cells even in the absence 

 of methods of genetic transfer. 



The occurrence of somatic crossing over (s.c.o.) in Drosophila has been demon- 

 strated by the classical work of Stern. 1275 This was based on the occurrence of twin 

 spots, when, on AbjaB individuals, the recessive phenotype aa appeared in an area 

 adjacent to one with the recessive phenotype bb. Stern's interpretation was that s.c.o. 

 has occurred between two of the four chromatids of the relevant homologous chromo- 

 somes. It was localized between the 5-locus and the kinetochore, and the next mitosis 

 led to sister cells with the genotype aBjaB and Ab/Ab. If the linked, recessive mutants 

 a and b, originally located on opposite members of a homologous pair of chromosomes, 

 were of a nature to produce a phenotype visible in the hypoderm when homozygous 

 (such as yellow and singed on the X chromosome which can be identified in single 

 bristles), then further multiplication of the crossover cells subsequently led to the forma- 

 tion of two adjacent patches of mutant tissue on a wildtype background. Somatic 

 crossing over was a much rarer event than crossing over in the germinal tissues. Its 

 frequency could be influenced by X irradiation, 68 ' 1386 by temperature, 138 and by 

 maternal aging. 139, 1179 While there seems to be no basic difference between meiotic 

 and mitotic crossing over, 1011 they appear to differ with regard to their response to modi- 

 fying factors. High temperatures decreased somatic crossing over in contrast to 

 germinal crossing over, which was increased. 1277 The agents that prevented meiotic 

 crossing over in male Drosophila did not affect somatic crossing over that occurs in 

 both sexes. 685, 1275 The C3G gene, which practically eliminated meiotic crossing over 

 in female Drosophila, had no effect on the frequency of somatic crossing over. 771 



