PHENOMENA OF INHERITANCE 271 



black. Very many factors are involved in the 

 production of white or black color but there is 

 at least one differential factor for every unit 

 character and this alone is the Mendelian fac- 

 tor. Of course there is no such thing as a 

 "sex-producing chromosome," sex being the 

 result of the interaction of many intrinsic and 

 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 large 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 



* See pp. 157 and 338. 



