GENETICS OF SOMATIC CELLS 421 



chromosome were lost or somatic reduction occurred, giving rise to haploid cells. 

 Somatic crossover could have been the explanation, provided that p was distal to c 

 and the crossover occurred between them, or that p was proximal to c and there was a 

 double crossover, or that/) and c were on opposite sides of the centromere with a crossover 

 occurring between them. Thus, while a few mechanisms by which the recessives 

 may come to express themselves in addition to somatic mutation can be ruled out, 

 others remain. 1117 Russell and Major consider somatic mutations or small or medium- 

 sized category deficiencies as most probably responsible. It may be added, as still 

 another category of phenomena, that although the occurrence of the change in hetero- 

 zygotes, its absence in homozygotes, and its inducibility by X rays strongly suggest 

 that it is determined at the genetic level, epigenetic phenomena 934 controlling genie 

 expression rather than the genetic information itself cannot be excluded with absolute 

 certainty. With these reservations in mind, together with the relative uncertainty 

 of the estimate due to the difficulty of obtaining an exact measure of the mosaic spots, 

 it is nevertheless interesting that the somatic rate of 7 x 10~ 7 /r/locus is of the same 

 order of magnitude as the germinal rate for the same four loci (2.4 x 10 -7 ). 1134 

 Analogous results have been obtained in Drosophila. 1 ' 1 * 



The most specific and regular kinds of somatic mutation and the most informative 

 studies concerning the genetic control of the mutation rate can be found in reports 

 on the genetics of maize and Drosophila. Rhoades has discovered that the a x gene 

 of the A x series of alleles in maize, although ordinarily very stable, can be made to 

 mutate at a high rate to other alleles in the series due to the presence of an entirely 

 different gene. 821 ' 1055 The A 1 series is located on the third chromosome, and when 

 the allele a x is present in homozygous form, no anthocyanin pigment is formed in the 

 aleurone of the endosperm or the plant. However, in the presence of a dominant 

 gene Dt, located on the ninth chromosome, it mutates to other alleles of the A x series 

 which are dominant to a x and allow the production of pigment. This mutation occurs 

 in the germ cells and the somatic tissues as well. In plants that are a^Dt-, colored 

 spots appear in the aleurone and narrow strips of pigment in the plant parts. During 

 development of the endosperm tissue the a± genes mutate to A u starting off centers of 

 growth of colored tissue which become visible as spots when they get large enough ; 

 each spot is assumed to arise from a single mutation. The fact that the spots are 

 usually of the same size indicates that the mutation takes place at a definite period of 

 development. Since the endosperm tissue involved is triploid, the effects of several 

 doses of the Dt gene can be compared. A single dose of the gene leads to 7.2 mutations 

 per seed, whereas the corresponding figures are 22.2 for double and 121.9 for triple 

 dose, respectively. 



Extensive information is also available about the genetic control of somatic muta- 

 tion in Drosophila. One example of this, the genetic control of somatic crossing over, 

 has been already discussed. The other outstanding example is somatic varie- 

 gation. 519 ' 786 ' im - 1172 ' 1173 This is similar to a mutational event, leading to somati- 

 cally mosaic phenotypes, and associated with a special type of chromosomal rearrange- 



