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 theré is in this instance 
the cytological observation to justify its use. It is, 
however, misleading to represent this 0 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 formule 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- 
