lO SEX-LINKED INHERITANCE IN DROSOPHILA. 



wise females with the formulas XX, XXY, and XXYY, that the Y is 

 without effect either on the sex or on the visible characters (other than 

 fertility) of the individual. 



The evidence is equally positive that sex is quantitatively determined 

 by the X chromosome — that twoX's determine a female and one a male. 

 For in the case of non-disjunction, a zero or a Y egg fertilized by an 

 X sperm produces a male, while conversely an XX egg fertilized by a 

 Y sperm produces a female. It is thus impossible to assume that the 

 X sperms are normally female-producing because of something else 

 than the X or that the Y sperm produce males for any other reason 

 than that they normally fertilize X eggs. Both the X and the Y sperm 

 have been shown to produce the sex opposite to that which they 

 normally produce when they fertilize eggs that are normal in every 

 respect, except that of their X chromosome content. These facts 

 establish experimentally that sex is determined by the combinations 

 of the X chromosomes, and that the male and female combinations are 

 the causes of sex differentiation and are not simply the results of male- 

 ness and femaleness already determined by some other agent. 



Cytological examination has demonstrated the existence of one 

 XXYY female, and has checked up the occurrence in the proper 

 classes and proportions of the XXY females. Numerous and extensive 

 breeding-tests have been made upon the other points discussed. The 

 evidence leaves no escape from the conclusion that the genetic excep- 

 tions are produced as a consequence of the exceptional distribution of 

 the X chromosomes and that the gens for the sex-linked characters are 

 carried by those chromosomes. 



MUTATION IN DROSOPHILA AMPELOPHILA. 



The first mutants were found in the spring of 1910. Since then an 

 ever-increasing series of new types has been appearing. An immense 

 number of flies have come under the scrutiny of those who are working 

 in the Zoological Laboratory of Columbia University, and the discovery 

 of so many mutant types is undoubtedly due to this fact. But that 

 mutation is more frequent in Drosophila ampelophila than in some of 

 the other species oi Drosophila seems not improbable from an extensive 

 examination of other types. It is true a few mutants have been found 

 in other Drosophilas, but relatively few as compared with the number 

 in D. ampelophila. Whether ampelophila is more prone to mutate, or 

 whether the conditions under which it is kept are such as to favor this 

 process, we have no knowledge. Several attempts that we have made 

 to produce mutations have led to no conclusive results. 



The mutants of Drosophila have been referred to by Baur as "muta- 

 tions through loss," but inasmuch as they differ in no respect that we 

 can discover from other mutants in domesticated animals and plants, 

 there is no particular reason for putting them into this category unless 



