SEX INHERITANCE 81 



number of the X chromosomes that are present. In 

 the eases 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 heterozygous for a 

 Mendelian factor for sex is derived from the fact that 

 he giA'CS 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 carrying 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 Mendehan 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 fertihzed by a male-producing sperma- 

 tozoon, contributes this factor to the resulting male. 



Although the only w^ay in which the results of sex 

 linked inheritance of the Drosophila type differ 

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

 namely, that a dominant male transmits his dominant 

 sex linked factor only to his- daughters, 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 



