CHAPTER IX 



SEX 



In their simplest expression the phenomena ex- 

 hibited by Mendelian characters are sharp and clean 

 cut. Clean cut and sharp also are the phenomena 

 of sex. It was natural, therefore, that a comparison 

 should have been early instituted between these two 

 sets of phenomena. As a general rule, the cross 

 between a male and a female results in the produc- 

 tion of the two sexes in approximately equal 

 numbers. The cross between a heterozygous domin- 

 ant and a recessive also leads to equal numbers of 

 recessives and of heterozygous dominants. Is it 

 not, therefore, possible that one of the sexes is 

 heterozygous for a factor which is lacking in the 

 other, and that the presence or absence of this factor 

 determines the sex of the zygote? The results of 

 some recent experiments would appear to justify 

 this interpretation, at any rate in particular cases. 

 An excellent illustration may be taken from poultry. 

 Many breeds of fowls are to be found in gold and 

 in silver varieties, the two birds differing essentially 

 only in colour. The gold-laced Wyandotte has 

 its counterpart in the silver-laced Wyandotte ; the 



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