NON-DISJUNCTION OF THE SEX CHROMOSOMES OF 



DROSOPHILA 



CALVIN B. BRIDGES 



The Zoological Laboratory, Columbia University 



A certain kind of exception that has come up during the course 

 of my work on Drosophila ampelophila, with Dr. T. H. Morgan, 

 bears directly upon the problem of sex determination, and espe- 

 cially upon the view that the chromosomes are the carriers of 

 the differentiators of the hereditary characters. The evidence 

 which I am to present deals with the X-chromosome^ of Droso- 

 phila, concerning which the evidence relating to sex linkage and 

 the linear arrangement of factors is more definite and complete 

 than in any other case. Ordinarily, when a female with white 

 eyes is mated to a wild male with red eyes, the daughters have 

 red, and the sons, white eyes. This is a typical case of the criss- 

 cross inheritance characteristic of sex linkage. The cytological 

 evidence of Miss Stevens shows that Drosophila belongs to that 

 group of forms in which the female has two X-chromosomes and 

 the male an unpaired X-chromosome. In spermatogenesis, half 

 of the spermatozoa receive this X and half do not. In oogenesis, 

 each gamete receives one X. The fertilization of any egg by a 

 one-X sperm, results in a female (X X"), while any egg fertilized 

 by a no-X sperm results in a male (X — ) . Morgan has explained 

 the case of criss-cross inheritance on the ground that the X- 

 chromosome is the carrier of all sex linked factors. The sons 

 are matroclinous because they receive their unpaired sex chromo- 

 some directly from the mother, and must show all the sex linked 

 characters which she showed. Each daughter, of the case cited, 

 is red because the paternal of her two sex chromosomes bears 

 the dominant red, and the maternal bears the recessive white. 



II have used 'X,' 'X-chromosome' and, sex-chromosome' interchangeably. 



587 



THE JOURNAL OP EXPERIMENTAL ZOOLOGY, VOL. 15, NO. 4 



