SEX INHERITANCE 93 



for supposing that maleness is due to one pair of 

 factors. It must be due to a complex of many factors 

 all of which are present in both sexes. 



2. Since there is evidence to show in some cases 

 that there is no sex factor in the Y chromosome, the 

 factor or factors carried by X can have no mate in 

 this sex, hence the allelomorph or allelomorphs must 

 be 0. If one chooses to represent this zero by a 

 small letter, by f or m for instance, there is no in- 

 consistency in doing so, for there is in this instance 

 the cytological observation to justify its use. It is, 

 however, misleading to represent this by M as has 

 sometimes been done. 



3. There is at present no evidence to show that 

 there is only one factor for sex carried by each sex 

 chromosome, however probable this may seem from 

 other relations, for it has not been possible to de- 

 termine the linkage relation of the sex factor or fac- 

 tors to other factors in the sex chromosome, because 

 crossing over of like factors in the homozygous sex 

 would lead to no visible effect, and in the heterozy- 

 gous sex no crossing over takes place. 



4. If in the formulae FF ( ?) and FO () the letter 

 F is interpreted as a factor for femaleness, the formula 

 must not be construed as meaning that F may not 

 also be a factor for maleness. For, as a matter of 

 fact, one F factor may be essential to the production 

 of the male. Therefore, until we get more definite 

 information as to the existence of a single or of 

 several factors for sex, and as to whether they are 

 the same factors in the two types, and what the rela- 



