GENETIC AND CYTOLOGICAL EFFECTS 207 



crossing over, and were found to be inviable as homozygotes or com- 

 pounds. These are attributes suggestix-e of deficiency, and all three 

 mutants wei'e identified as deficiencies by f2;enetic evidence showing that 

 the induced alteration in each case involved loss of the effects not only of 

 A but of additional genes affecting chlorophjdl de\'elopment and viability. 

 The four ultraviolet mutants gave no indication of deficiency by any of the 

 criteria mentioned. 



The evidence for Drosophila comparing the genetic effects of X-ray and 

 ultraviolet radiation contrasts somewhat in its general implications with 

 that from plants. It would not be surprising to find actual contrasts, 

 but the evidence from both sources is still too scanty to force the assump- 

 tion of any basic difference in the nature of the chromosomal or genie 

 alterations induced. The contrasting indications are briefly noted as 

 follows : 



1. Gross chromosomal rearrangements. Asshownby Altenburg (1934) 

 and by Muller and Mackenzie (1939), there is no appreciable frequency 

 of gross chromosomal rearrangements in ultraviolet progenies which yield 

 an abundance of sex-linked lethals. Demerec et al. (1942) found a single 

 translocation in the progeny of ultraviolet-treated flies, but this single 

 case cannot be considered evidence of an effect of the treatment. The 

 absence of induced translocations in the Drosophila cultures tested does 

 not represent a conflict in the evidence, for there is no necessary implica- 

 tion that the radiation is unable to induce translocation in low frequency. 

 The Drosophila evidence shows that the ratio of induced translocations to 

 induced sex-linked lethals is far lower with ultraviolet than with X rays. 

 The maize evidence also shows that the ratio of translocations to muta- 

 tions is far lower than with X rays, and shows further that translocations 

 are induced in very low frequency. 



A search for terminal deficiencies of the X chromosome, identifiable by 

 genetic markers, was made by Mackenzie and Muller (1940) and by 

 McQuate (1950), and none was found. The occurrence of ultraviolet- 

 induced terminal deficiencies in maize was shown cytologically by De Boer 

 (1945). 



2. Nature of the induced mutations. Although genetic experiments to 

 detect minute rearrangements, by Mackenzie and Muller (1940), indi- 

 cated that they were absent or very rare in ultraviolet progenies, the direct 

 cytological study of induced lethals by Slizynski (1942) clearly showed 

 that short deletions are included in this class. The frequency of cytologi- 

 cally detectable deletions among the sex-linked lethals was not much lower 

 among the ultraviolet than among the X-ray cases. They occur also, in 

 considerable frequency, among sex-linked lethals arising in untreated 

 material. 



The implication from the evidence in maize is that the ultraviolet 

 mutants are distinctlv different from the X-rav mutants in that the 



