144 Applied Biophysics 



Discussion of the nature of the dominant lethals is deferred, 

 except to indicate that the change in the heredity material does 

 not produce an immediate effect. Eggs which fail to hatch are 

 found to have undergone a number of nuclear divisions before 

 breakdown occurs. ^^ 



Among the viable offspring of treated male flies, a number 

 carry mutations. The great majority of these are recessive, and 

 so do not produce any visible effect immediately, since they are 

 heterozygous. Special measures have to be taken to obtain 

 individuals homozygous for such mutations. The simplest are 

 those for detection of mutations in the X-chromosome, a sex 

 chromosome that is present twice in the female flies and once 

 only in the males. It crosses and recrosses in heredity in a 

 regular fashion from father to daughter and mother to son. Thus, 

 males will be hemi-zygous for genes in the X-chromosome, and 

 so will manifest them. 



Treated male parents are mated to C I B females,^^ one of 

 whose X-chromosomes carries a cross-over suppressor (C, ac- 

 tually an inversion), a recessive lethal (/), and a dominant 

 marker-gene (B, Bar-eye, which is narrower than the normal 

 round eye). Among the offspring, females with a Bar-eye are 

 chosen and mated individually with any suitable males, prefer- 

 ably with their X-chromsomes suitably marked wnth recessive 

 genes. Any one of these Fi females will have a treated X-chromo- 

 some from her father and 3. C I B chromosome 'from her mother. 

 The C I B chromosome will be lethal to male offspring carrying 

 it, so all male offspring of Fi females will carry only treated 

 X-chromosomes from their grandfathers. Inspection of these 

 males will disclose genes having a visible effect, though their 

 detection will depend on the skill and experience of the observer. 

 On the other hand, if a recessive mutation is lethal, the culture 

 containing it will l)e marked by a complete lack of male offspring. 

 Such sex-linked lethals are ])roduced by radiations about ten 

 times as frequently as visible mutitions. They provide an objec- 

 tive criterion for quantitative work, and have been widely used 

 in experimental studies on mutation-rates. The recessive lethals, 

 of course, represent nuitations at a large number of different loci, 



