512 RADIATION BIOLOGY 



the nonlethal chromatid, would hve. However, their daughters which 

 had received the lethal-bearing chromosome would be recognized as such 

 when tested. Hence the lethal would be recorded as not having arisen 

 until the first generation after treatment. In the individuals of the 

 second generation after treatment this complication could no longer 

 play a role, and in that generation no significant increase in the frequency 

 of origination of mutations was found. Admittedly, however, the 

 interval involved is so long that the test is far from a delicate one. 



According to later tests of a similar kind, carried out on a larger scale 

 by Timofeeff-Ressovsky (1931b, 1937b), viable males containing an 

 X chromosome which had been irradiated in the previous generation 

 (derived by crossing irradiated males to females with attached X chromo- 

 somes) showed no heightened frequency of origination of sex-Unked 

 lethals whatever. This might seem contradictory to what has just been 

 said concerning fractionals. However, unlike what had occurred in the 

 earlier work on this subject, in which the females of the first generation 

 after treatment (derived by crossing irradiated males to females with 

 separate X chromosomes) had been tested, many fractional cases in this 

 experiment must have been automatically eliminated by the genetic 

 method used, whereby the individuals of the first generation after treat- 

 ment which carried the irradiated X chromosomes were males. For a 

 sex-linked lethal, even if carried by only half the body, might be expected 

 often (although not always) to kill the male containing it. This killing 

 would be less and less likely to occur, the later in the course of the embry- 

 onic or postembryonic life of the male the lethal had arisen, although 

 there are some effects which are even cell lethal, as noted in Chap. 7. 

 These results with X rays, indicating the absence or rarity of a 

 mutagenic aftereffect, form a contrast with those obtained by Auerbach 

 (1949) with mustard since, after treatment with this chemical, there was a 

 definite persistence of mutagenicity, in certain loci at any rate, for as 

 long as two generations after treatment. 



The previously cited experiments, indicating that ionizing radiation 

 does not produce long postponed mutations, were on a coarse time scale 

 and dealt chiefly with the possibility of effects on later generations of 

 individuals, separated from the irradiation by days or weeks, rather than 

 on later cell generations, separated by hours or days. Evidence on a 

 finer time scale is provided by examination of the immediate offspring of 

 irradiated individuals to determine whether they exhibit a heightened 

 frequency of phenotypic effects that are confined to a small portion of 

 the somatic tissues. This is a result to be expected if visible mutations 

 or losses of chromatin had been produced as aftereffects in embryonic or 

 larval somatic cells, several cell generations removed from the irradiated 

 parental germ cell. In Drosophila males derived from irradiated 

 mothers having separate X chromosomes or from irradiated fathers that 



