108 SEX 



fertilised egg-cells will bear only the dominant 

 character, 50 per cent, will bear both the 

 dominant and the recessive character (only 

 the dominant being eventually expressed or 

 well-expressed), and 25 per cent, will bear only 

 the recessive character. This is the theory 

 of the segregation of pure gametes and it is 

 the central conception of Mendelism. A 

 scheme will make it clear. (See Evolution, 

 p. 135.) 



1 pure dominant. 



2 impure dominants. 

 1 pure recessive. 



When an organism produces only one kind 

 of germ-cell, as regards a particular character, 

 it is said to be homozygous ; when it produces 

 two kinds of germ-cells, some bearing the 

 factor of a given character, and others not, 

 it is said to be heterozygous. Thus in our 

 previous scheme the forms labelled D(R) 

 are heterozygous, while those labelled D or 

 R are homozygous. 



If sex, like any other character, is due to 

 some item or factor or determiner in the 

 chromosomes of the germ-cells (situated, it 

 may be, along with sex-linked characters, 

 in the sex chromosome already referred to), 

 there are three theoretical possible Mendelian 

 interpretations. Both males and females 



