RADIATION AND THE STUDY OF MUTATION IN ANIMALS 1213 



111 this method, tho presence of a new lethal is detected by a progeny 

 consisting entirely of females (Fig. 1). 



Autosomal lethals present a somewhat more difficult problem, 

 although the difference is one of detail (Fig. 2). They allow a further 

 elaboration of technique, which is not possible with sex-linked lethals. 

 Since both sexes may carry an autosomal lethal, it is possible by appro- 

 priate crosses to carry, in "balanced" stocks, chromosomes which are 

 allowed, generation after generation, to accumulate lethals. At the end 

 of a stated period tests are made, and the occurrence of new lethals 

 determined. This method presents in acute form the difficulty met with 

 in all lethal mutation work that frequently two lethals which are closely 

 linked may be mistaken for one. 



The lethals so far dealt with are the recessive lethals. There is a class 

 of dominant lethals, whose presence in single dose causes the death of 

 the zygote. They are detected by departures from expected ratios in 

 certain matings, and while they are probably largely due to chromosomal 

 aberrations, are here mentioned because of their high frequency of 

 detection in certain special cases (43, 54, 58, 105, 148, 167). 



It is readily understandable that these techniques used in Drosophila 

 do not, in principle, depend on special attributes of the species. Their 

 generality is a function of the chromosomal mechanism on which they 

 are based. It may be anticipated that mutation studies on any animal 

 must be aided by a thorough study of the methods developed by Muller 

 (see especially 107). Indeed, this has already been indicated by the 

 studies of Snell (162) on mice and of AstaurofT (6) on the silkworm. 



Aside from technique, there is a question of definition, which must 

 be considered here. It is this: the criteria which distinguish the mutation 

 of genes from chromosome abnormalities are far from sharply defined. 

 Chromosomal aberrations of various types have long been known 

 (Bridges, 9 ; see also Mohr, 88) to produce character changes which might 

 have been attributed to gene mutations. As Stadler (166) has pointed 

 out, gene mutations form a residue, which includes those cases where no 

 gross chromosomal abnormality can be demonstrated. This purely 

 negative definition, while it has raised many theoretical difficulties, 

 suffices in practice ; that is to say, in planning experiments. Meanwhile, 

 the recent work of Painter (128) on the chromosomes of the salivary 

 glands of Drosophila raises hopes that presently some more definite 

 criteria can be set up. 



THE CHARACTERISTICS OF THE MUTATION PROCESS 

 THE DIVERSITY OF MUTATION RATES AT DIFFERENT LOCI 



At the outset, experiments on mutation rate meet the difficult prob- 

 lems which populations present. Either a frequently occurring phenotype 

 is selected — the lethal, for example — and the rate of its occurrence 



