CHAPTER XXXIV 
SEX IN ANIMALS 
Sex-determination with its attendant problems has always been a 
subject of great interest to practical animal breeders; and the art of 
breeding has not lacked rules by which the sex ratio might be shifted in 
various ways to the advantage of the breeder. But most of these rules, 
like many beliefs current at one time or another in animal breeding have 
been founded upon inadequate evidence or unsound reasoning. Sex 
remains a matter beyond the control of the breeder: its ultimate control 
is entirely problematical. 
The Determination of Sex.—The thesis that sex is determined at the 
time of fertilization has been elaborated fully in Chapter XI. It was 
pointed out there that sex, like other characters of the individual, has 
a definite factorial basis, that the factorial constitution of the individual 
with respect to sex as well as to other characters is fixed by the constitu- 
tion of the two gametes which unite to form the zygote. There is 
every reason to believe that sex is determined in this same fashion in 
domestic animals, at the time of fertilization; and that any treatment 
subsequent to that time cannot affect the sex of the individual. At least 
this much may be said, that any theory of sex-determination in the higher 
animals which is based upon other factors than chromosome constitution, 
must be brought into harmony with the known facts of the chromosome 
relations in sex-determination. 
Sex-determination in Mammals.—It appears to be fairly well estab- 
lished that the inheritance of sex in mammals always is of the XY type, 
that is the females are homozygous for a determiner of femaleness whereas 
the males are heterozygous. Since this group includes practically all 
domestic animals, except the feathered ones, it follows that in horses, 
cattle, sheep, goats, swine, etc., the mode of inheritance of sex is of this 
type. The direct evidence for this conclusion in domestic animals is 
exceedingly meager, but the main outlines are sufficiently clear to provide 
fairly satisfactory confirmation of this general conclusion. 
For direct cytological evidence of the mode of determination of sex 
in these domestic animals we are indebted particularly to the extensive 
investigations of Wodsedalek. These investigations do not provide a 
complete body of evidence, but they indicate very strongly that unequal 
distribution of chromosomes takes place in the male in the horse and 
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