PHENOMENA OF INHERITANCE 265 



extrinsic causes. The X chromosome is only 

 one factor in the determination of sex, but 

 if it is a factor which differs in the case of 

 the two sexes it is a "sex determining factor." 

 There are many parts of a germ cell, all of 

 which may be concerned in heredity and de- 

 velopment, but the chromosomes appear to be 

 the seat of the differential factors for Men- 

 delian characters. 



2. Modifications of the Principle of Domi- 

 nance. A great number of animal and plant 

 hybrids show one contrasting character com- 

 pletely dominant over the other one as Mendel 

 observed in the case of his peas. But in a 

 considerable number of cases this dominance 

 is incomplete or imperfect. When white- 

 flowered strains of four o'clocks are crossed 

 with red-flowered ones the F x plants bear 

 neither white nor red flowers but pink ones, 

 and the F 2 plants bear white, red and pink 

 flowers. The whites and reds are always 

 homozygous, the pinks heterozygous; pure 

 white and pure red are produced only when 

 their factors are duplex (WW), (RR) ; when 

 they are simplex (WR) pink is produced. In 



