ON THE RELATION OF RACE CROSSING TO 

 THE SEX RATIO. 



MAUD DeWITT pearl AND RAYMOND PEARL. 



Introduction. 



There would appear to be widely prevalent among practical 

 stock breeders an opinion that the relative proportion of the sexes 

 may be influenced by the method of breeding practiced. As 

 evidence of the existence of such an opinion two citations will 

 suffice. Others might be given. Davenport in his memoir on 

 "Inheritance in Poultry," ^ introduces a section on "Sex in Hy- 

 brids " (p. 97) with the statement that : " There is a widely held 

 and frequently expressed opinion that hybrids show an excessive 

 proportion of males." He further says that: " Bateson and 

 Saunders probably have this in mind in their statement — ' the 

 statistical distribution of sex among first crosses shows great 

 departure from the normal proportions.' " No support is given 

 to the view that hybrids show an undue proportion of males by 

 Davenport's own statistics, the general conclusion being that : 

 " The exceptions to the law of equality of sexes in hybrid off- 

 spring are . . . individual and not of general significance." 



It is a matter of interest to note that while the opinion appears 

 to be widespread that the kind of breeding practiced influences 

 the sex ratio there is not entire uniformity as to what the influence 

 of a particular method of breeding on sex is. Thus one would 

 infer from a statement in a recent work by Miiller^ that it has 

 been generally held by continental breeders, at least, that inbreed- 

 ing tends toward the production of an unduly large proportion 

 of males. Miiller^ {loc. cit.) in discussing the experiments of 

 Schultze {of. infra) makes the following statement concerning 

 certain of that author's results : " Das Verhaltnis der beiden 

 Geschlechter war vielmehr bei strengster Inzucht (Paarung nur 



1 Carnegie Institution of Washington, Publication No. 52, 1906. 



^'Miiller, R. " Biologic und Tierzucht." Gedanken und Tatsachen zur biolo- 

 gischen VVeiterentwicklung der landwirtschaftlichen Tierzucht. Stuttgart (Ferd. 

 Enke), 1905. Pp. 96. 



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