Point Mutants — Detection and Effects 



205 



Such a lethal mutant must have occurred in 

 the germ line after the fertilization that pro- 

 duced the Pi male, for if it were present at 

 fertilization, he would not have survived. It 

 is unlikely that many of the lethals detected 

 in sperm originate very early in development, 

 for in this case a large portion of the somatic 

 tissue would also carry the lethal and this 

 would usually cause death before adulthood. 

 Usually, the X-linked lethal present in sperm 



arises in a small portion of the germ line so 

 that even when a few hundred sperm, of the 

 several thousand sperm ejaculated, are tested, 

 only one is found to carry a mutant. Occa- 

 sionally, however, the mutation occurs early 

 enough in the germ line so that several sperm 

 tested from the same male carry what proves 

 to be the same recessive lethal. 



When a thousand sperm from normal un- 

 treated males are tested for X-linked reces- 



FIGURE 24-1. The breeding scheme used in the Base technique. 



\--x. , -Base 



■i c 



Base 



Base 



>== I 



Base 



c/ 



Base 



9 



Base / 



I ^9 X 



Base 



Base (Individually) 



Base 



9 



9 



Base 



?c/ 



O (11 



I n 



S^S O <2) 



(1) is absent if the F^ Base ehromosome contributed to the Pj L> 

 contained a recessive lethal • 



(2) is absent if the f^ + chromosome contributed to the Pj O 

 contained a recessive lethal T 



/\lK^ 



