i62 BREEDING 



to the true one. And it is given here partly for 

 its general biological interest, and partly because it 

 is true up to such a point that it is possible, by means 

 of it, to predict the results of all the simpler types 

 of matings which the practical breeder is likely to 

 want to make. 



In the previous chapter we were confronted with the 

 fact that there must be something in the male repro- 

 ductive cell, or germ cell, as it is called, contained 

 in the pollen grain taken from a flower on a tall 

 plant, which made the offspring of a dwarf plant 

 fertilised by this pollen, develop into a tall plant. 

 Mendel's theory is that every germ cell, whether 

 male or female, of a tall plant contains something 

 representing the character tallness. It is not known 

 what this something is, so it does not matter what 

 it is called. The term that has come into general 

 use is " factor." The germ cells of tall peas are, 

 therefore, said to contain the factor for tallness. 

 Similarly the germ cells of a dwarf pea contain 

 the factor for dwarfness. So that when a tall pea 

 is self-fertilised, a male germ cell containing the factor 

 for tallness unites with a female germ cell also con- 

 taining the factor for tallness, and the result of this 

 double dose of tallness in the fertilised ovule is that 

 it develops into a tall pea. Similarly, when a dwarf 

 pea is self-fertilised a male germ cell containing 

 the factor for dwarfness unites with a female germ cell 

 also containing this factor, with the result that a dwarf 

 pea is produced. In both cases, it will be noted, two 

 doses of the same factor are necessary for the pro- 



