A DEMONSTRATION OF GENES MODIFVIXCr TIIK 



CHARACTER "NOTCH." 



By T. H. Morgan. 



Two main topics are dealt with in the following pa^cs fnjin the 

 standpoint of the experimental results obtained. One of them con- 

 cerns the demonstration of modifying genes that were involved in the 

 results of a selection experiment. The other topic is a discussion of 

 the possibility of contamination of genes as a method that luus l>een 

 appealed to as an influence vitiating the regularity of Mendelian 

 phenomena. 



The claim of the Mendelians that genes have been found to l>e sta- 

 ble in successive generations wherever a critical test of them was made 

 has been challenged both on the grounds of empiric observation and 

 on the more sentimental grounds that such hard and fa^t rules do not 

 apply to living things which are rather to be thought of as variable 

 quantities. In the following pages an account is given of a character 

 that changed in the course of selection and a demonstration that the 

 result was due to a modifying gene and not to contamination between 

 the notch gene and its normal allelomorph, despite the fact tliat an 

 exceptional opportunity was given to contaminate the gene, if contami- 

 nation is a possible process. 



In 1915, Dexter described a mutant type of Drosophih called Notch 

 or "perfect Notch," and made out the main points in the heredity of 

 the character. The gene is sex-linked, and dominant for the serra- 

 tion that it produces in the wings, but recessive in its lethal effect. 

 Since the gene is carried by the X chromosome, any male that gets a 

 chromosome with this gene dies, while the female that has another X 

 carrying the normal allelomorph lives and shows the notch at the end 

 of her wings (fig. 91). Since no male that has the notch gene can live, 

 it is not feasible to determine whether a female containing two letluil 

 bearing X's would also die.^ Every heterozygous Notch fcnuile gives 

 twice as many daughters as sons, because, as stated, half of the sons 

 die, namely, those that get the lethal-bearing X". The scheme is iis 

 follows : 



X-X" eggs 



X — Y sperm 



XX — Xr* — XY — X^Y (dies) 

 9 9 cf d" 



1 Unless an XX egg, arising through "equational non-di.sjunrtion," were fcrtilued by a Y 

 sperm and lived. No such females have appeared. They would have no rcRuinr i»on9. 



345 



