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- 

 dehan characters. 



2. Modifications of the Principle of Domi- 

 nance. — ^A great mmiber 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 Fi plants bear 

 neither white nor red flowers but pink ones, 

 and the F2 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), (BR) ; when 

 they are simplex (WB) pink is produced. In 



