MANNER OF PRODUCTION OF MUTATIONS 513 



had been crossed to females having attached X chromosomes, any sex- 

 hnked gene mutation or structural change having a recessive visible 

 effect and any dominant visible change in any chromosome become evi- 

 dent in those body regions containing them which are appropriate for 

 their manifestation, and the resultant individual is termed a mosaic. At 

 the same time, female offspring of an irradiated parent of either sex mani- 

 fest any dominant visible change. The size of the patch of abnormal 

 tissue gives an indication of the approximate cell generation in which the 

 given genetic change occurred. 



It has already been explained that many structural changes produced 

 by breaks in irradiated spermatozoan chromosomes of Drosophila are 

 not completed until after chromatid formation and that the broken ends 

 of the sister chromatids may then join in different ways. Approximately 

 half-and-half mosaics are thereby produced, when the structural change 

 undergone by one of the chromatids has a visible manifestation either by 

 reason of position effect or aneuploidy. Occasionally, union of some of 

 the parts may be delayed even until after the second division of the 

 irradiated chromosome (as indicated, for instance, by the evidence from 

 gynandromorphs), and this might, if an acentric piece were still available 

 for union, result in a mosaic having one or more different quarters instead 

 of halves. The pattern of distribution at these early mitoses cannot be 

 determined with exactness, however, for three main reasons: (1) the 

 appropriate tissues or parts for manifesting a given visible effect are often 

 very limited ones, (2) there is much indeterminism in the distribution of 

 early "cleavage" nuclei among different parts of the body, and (3) nuclei 

 or cells with some types of structural changes are probably subject to 

 more or less retardation in their proliferation, with the result, for example, 

 that one of the first two nuclei may give rise to only about a third of the 

 body (Bonnier et al, 1949). 



In view of these sources of variation it is noteworthy that even the 

 first observations on visibly mosaic mutants obtained from irradiated 

 spermatozoa (Muller, 1927, 1928b) indicated that the great majority 

 were probably of half-and-half derivation but that some may have been 

 quarter mosaics. It is true that later work on this subject by Moore 

 (1934) gave results which were irreconcilable with only an immediate 

 mutational effect. According to his data, as large a proportion of his 

 visible mutants were mosaics even when the parent had been irradiated 

 in a larval stage or when oocytes had been irradiated as when spermatozoa 

 had been irradiated, and under all circumstances there seemed to be a 

 high proportion of mosaics having only a small fraction of abnormal 

 tissue. However, there are several reasons for questioning the reliability 

 of these data, and they are in disagreement with those obtained both 

 previously and subsequently. Thus, Timofeeff-Ressovsky (1937a, b) 

 found that, when spermatozoa were irradiated, the abnormal tissue (as 



