CHAPTER IV 



HEREDITY AND SEX 1 



The value of a domesticated animal often depends in 

 considerable measure on its sex. Therefore, if a means 

 could be devised for controlling the sex of offspring, it 

 would be of great economic value to the breeder. Endless 

 attempts have been made to do this, and occasionally a 

 claim of success has been made, but none of these claims 

 has withstood the test of critical analysis or experiment. 

 The hypotheses advanced to explain how sex may be con- 

 trolled have been of the most varied character. In some 

 the determination of sex has been supposed to inhere in the 

 nature of the parents, in others it is referred to conditions 

 of the gametes themselves. 



Relative age or vigor of the parents has been supposed 

 to influence sex in various ways. The same idea has been 

 advanced regarding the gametes themselves, it being 

 supposed that early or late fertilization of the egg might 

 influence its sex. Experimental evidence, however, as to 

 these several hypotheses is wholly negative, when one elimi- 

 nates other possible factors from the experiment. Every- 

 thing points to the conclusion that sex rests in the last 



1 In reading this chapter, the following definitions should be kept in mind. 



Gamete: a mature reproductive cell capable, on uniting with another gamete, 

 of forming a new individual. 



Zygote: the new individual formed by the union of two gametes. 



Homozygote: an individual formed by the union of two gametes of like char- 

 acter as regards heredity 



Heterozygote: an individual formed by the union of two gametes of unlike 

 character as regards heredity. 



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