Detection of Autosomal Lethals 255 



ticular stock from Oregon, 0.07 per cent. Other investigators 

 have observed differences in other strains. It is interesting to 

 note that different investigators obtained the same mutation 

 rate for chromosome I from Florida stocks and that if the muta- 

 tion rate of chromosome I is high (or low) in a given stock, the 

 rate of mutation in chromosome II is also high (or low) for the 

 same stock. Different strains of Drosophila differ significantly 

 in the rate at which mutations occur. This fact must be consid- 

 ered whenever studies are made on the effect of environmental 

 differences on rates of mutation. Another factor that must be 

 considered is the elimination of spontaneous lethal mutations by 

 selection. If, over a period of several generations, those lines 

 are repeatedly discarded which show lethals while the lines which 

 show the most offspring are used for continuing the stock, the 

 percentage of lethals will be considerably less in these lines than 

 in unselected families of the same stock. Selection will definitely 

 decrease the percentage of spontaneous lethals. 



Detection of Autosomal Lethals 



The method of detecting recessive lethal mutations which are 

 induced in autosomes is somewhat more laborious than the 

 method used to detect induced lethals in the X chromosome. 

 This is because autosomal lethals must be made homozygous 

 before they can be detected whereas sex-linked lethals can be 

 detected in the male even though only one such gene is present 

 since the Y chromosome does not bear the nonlethal allele. In 

 the method first indicated by Muller but set up by Child (de- 

 scribed by Plough, 1941), a wild-type stock is raised at the 

 desired temperature and is then mated with a marker stock (Fig. 

 77). The marker chromosomes bear certain dominant genes for 

 identification and crossover suppressors to prevent a transfer of 

 an induced lethal from a treated to an untreated chromosome. 

 A stock frequently used has one member of chromosome II with 

 a gene for Curly wings (Cy) and a crossover suppressor (C) and 

 a homologue with the gene for Star eye (S) ; one member of chro- 

 mosome III has a crossover suppressor (C) plus the genes Sb 

 (Stubble bristles) or D (Dichaete wings), whereas the other 

 member has such genes as H (Hairless) or Dfd (Deformed). 

 All the genes except the crossover suppressors are lethal when 

 homozygous so that CyCy SbSb stocks cannot be used. 



