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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