HEREDITY IN SOMATIC CELLS 359 



Nothing similar has been observed in other chromosomes, perhaps for the 

 reason that they are not as easy to observe. 



If these phenomena are correctly interpreted and of general occurrence, 

 there is still the question of how they are controlled. Whether some 

 extrachromosomal condition brings about this chromosomal behavior, or 

 whether it is achieved by the action of chromosomal controlling agents, 

 we cannot say. But we do know, as should be evident from the contents 

 of the earlier chapters in this book, that there are a number of other ways 

 in which somatic variation may be achieved. Mutation of chromosomal 

 genes and selection, mutation and segregation of nonchromosomal factors 

 with autonomous action, mechanisms of infectious heredity including 

 viruses, and the formation of phenocopies, all provide food for thought 

 and experimentation. Perhaps it will be shown that some of these 

 processes do not have the orderliness necessary for organized develop- 

 ment; they are still of interest as possible explanations of abnormal 

 growth and the degenerative changes accompanying senescence. But 

 even more important, their study will reveal the hereditary mechanisms 

 which operate in somatic cells, in terms of which problems of normal and 

 abnormal development must find their solution. Each of these mecha- 

 nisms will be taken up in turn; first we will deal with a phenomenon of 

 genetic exchange in somatic cells, alternatively called somatic or mitotic 

 crossing over. 



RECOMBINATION IN SOMATIC CELLS 



It was long ago shown that genetic exchange can occur during mitosis 

 by the process of crossing over. In making his beautiful demonstration 

 of this fact. Curt Stern used Drosophila, heterozygous for the recessive 

 genes singed (sn), which curls the bristles, and yellow (y), which colors 

 the cutaneous tissue yellow instead of grey. These genes are both 

 located on the X chromosome, as shown in Figure 12.2. The hetero- 

 zygous flies showed mosaic patches of different sorts. The most frequent 

 was a twin spot, in which a small patch of yellow tissue bordered on 

 a patch with singed bristles. The next most frequent mosaic showed 

 only a small patch of yellow tissue in the midst of a background of wild- 

 type cells. Finally, in low frequency, mosaics with single singed-bristle 

 spots were found. These results are explained in terms of an exchange 

 between the homologous chromosomes, which can in fact be observed to 

 be paired during the prophase of mitosis in the somatic cells of Dro- 

 sophila. The centromeres of the four chromatids involved in the exchange 

 later separate in the usual mitotic fashion, a and b, like c and d, moving 



