INTRODUCTORY. 9 



Ordinarily all the sons and none of the daughters show the recessive 

 sex-linked characters of the mother when the father carries the domi- 

 nant allelomorph. The peculiarity of non-disjunction is that some- 

 times a female produces a daughter like herself or a son like the 

 father, although the rest of the offspring are perfectly regular. For 

 example, a vermilion female mated to a wild male produces vermilion 

 sons and wild-type daughters, but rarely also a vermilion daughter 

 or a wild-type son. The production of these exceptions (primary 

 exceptions) by a normal XX female must be due to an aberrant reduc- 

 tion division at which the two X chromosomes fail to disjoin from each 

 other. In consequence both remain in the egg or both pass into the 

 polar body. In the latter case an egg without an X chromosome is 

 produced. Such an egg fertilized by an X sperm produces a male with 

 the constitution XO. These males received their single X from their 

 father and therefore show the father's characters. While these XO 

 males are exceptions to sex-linked inheritance, the characters that they 

 do show are perfectly normal, that is, the miniature or the bar or other 

 sex-linked characters that the XO male has are like those of an XY 

 male, showing that the Y normally has no effect upon the development 

 of these characters. But that the Y does play some positive role is 

 proved by the fact that all the XO males have been found to be abso- 

 lutely sterile. 



While the presence of the Y is necessary for the fertility of the male, 

 it has no effect upon sex itself. This is shown even more strikingly by 

 the phenomenon known as secondary non-disjunction. If the two 

 X chromosomes that fail to disjoin remain in the egg, and this egg is 

 fertilized by a Y sperm, an XXY individual results. This is a female 

 which is like her mother in all sex-linked characters (a matroclinous 

 exception), since she received both her X chromosomes from her mother 

 and none from her father. As far as sex is concerned this is a perfectly 

 normal female. The extra Y has no effect upon the appearance of the 

 characters, even in the case of eosin, where the female is much darker 

 than the male. The only effect which the extra Y has is as an extra 

 wheel in the machinery of synapsis and reduction; for, on account of 

 the presence of the Y, both X's of the XXY female are sometimes left 

 within the ripe egg, a process called secondary non-disjunction. In 

 consequence, an XXY female regularly produces exceptions (to the 

 extent of about 4 percent). A small percentage of reductions are of 

 this XX-Y type; the majority are X-XY. The XY eggs, produced by 

 the X-XY reductions, when fertilized by Y sperm, give XYY males, 

 which show no influence of the extra Y except at synapsis and reduc- 

 tion. By mating an XXY female to an XYY male, XXYY females 

 have been produced and these are perfectly normal in appearance. 

 We may conclude from the fact that visibly indistinguishable males 

 have been produced with the formulas XO, XY, and XYY, and like- 



