SEX INHERITANCE 81 



number of ^he X chromosomes that are present. In 

 the cases that follow, where sex determination of the 

 Drosophila type was discovered by a study of sex 

 linked inheritance, as well as in the above cases, where 

 the mechanism was discovered through cytological 

 observations, proof that the male is heterozj^gous for a 

 Mendelian factor for sex is derived from the fact that 

 he gives rise to two kinds of spermatozoa — male pro- 

 ducing and female producing — in equal numbers. 

 We know this in the cases worked out cytologically 

 because here the spermatozoa carrjdng X must all 

 produce females, while the other half must produce 

 males ; and we know it, in the cases worked out gen- 

 etically, because here only half the spermatozoa from 

 a male with a dominant sex linked character carry 

 the dominant factor, and these all produce females, 

 while the rest produce males. The female must con- 

 tain the same Mendelian sex factor as is present in 

 the female-producing spermatozoa of the male ; but 

 the female must be homozygous for this factor, since 

 any egg, if fertilized by a male-producing sperma- 

 tozoon, contributes this factor to the resulting male. 



Although the only way in which the results of sex 

 linked inheritance of the Drosophila type differ 

 from non-sex linked cases is the one above stated, 

 namely, that a dominant male transmits his dominant 

 sex linked factor only to his daughterly nevertheless 

 it may be well at this point to recall specifically what 

 ratios are produced in consequence, in the various 

 types of crosses. 



Examples of sex linked inheritance in Drosophila 



