GENES MODIFYING NOTCH. 



381 



LITTLE EYES. 



There appeared in the SS AAA 3346262 generation of the selected 

 stock some mutant flies that had not only the wings something like 

 those of Notch, but the eye was also of tenreduced (plate 1 2 , a, 6, c) . Since 

 the latter condition had been also found occasionally in short stock, 

 the occurrence here of this new type, called little eyes, suggested the 

 possibility that a new allelomorph of Notch had appeared. The sequel 

 shows the futility of any such judgment in regard to the gene based 

 on the appearance of the character. The new mutant proved to be so 

 weak, so inviable, and so infertile that almost nothing could be done 

 with it. It could, in fact, only be kept in existence by large mass- 

 cultures of flies known to contain the genes. The females never bred, 

 although many attempts were made to breed them. A few males 

 mated to ruby females gave offspring, and these FI flies gave, along 

 with many normal offspring, a few small-eyed flies of both sexes. The 

 numbers were very small, but as both males and females were present, 

 the result shows that the character is not sex-linked and therefore 

 that it can not be an allelomorph of Notch. The location of the gene 

 in its chromosome was not made out because the stock died. It 

 will be noted that two of the females figured have * M 

 Notch-like wings, while the other female and the male 

 have rounded wings. It is probable that the two 

 females really had the Notch gene, since the mutant 

 arose in the stock, but other females were not Notch, as 

 shown here and as frequently observed in later cultures. 

 There is no evidence that any males were notched, , 

 although the beading might closely resemble notching. 

 Great variability of the character was observed in 

 fact, some individuals could be detected only by the 

 very slightly smaller eye or a tendency for the wings 

 to spread out. 



HIGH SEX-RATIOS CAUSED BY LETHALS. 



Notch is a recessive lethal, and if by chance another 

 lethal had been present in the X chromosome from the 

 father of such a female, all of her sons would die except 

 the occasional son due to crossing-over between the 

 lethal factors. For instance, if the Notch gene has 

 the location shown in figure 101 and another lethal 

 factor in the other chromosome located as shown in the FIG. 101. 

 same figure, then either chromosome that goes into an egg that is later 

 fertilized by a Y-bearing spermatozoon will die, but by crossing-over 

 between the Notch and the lethal loci there will be produced one chro- 

 mosome bearing two lethals, and another with their normal allelo- 



X 



