MENDELISM 123 



The same in diagram form : 



D R 



P 



D(R) F, 



DD D(R) D(R) RR 



FIG. 49. MENDELISM. (After Thomson.) 



Here DD stands for pure, or so-called extracted, domi- 

 nants, RR for pure recessives, and D(R) for impure 

 dominants. 



We see, then, that when two distinct varieties are crossed, 

 where one is dominant with regard to a certain character, 

 while the other is recessive, the first hybrid generation (Fj) 

 is an impure dominant. On interbreeding, the next 

 generation (F 2 ) can be divided into four parts one pure 

 dominants, two impure dominants, and one pure recessives. 

 The impure dominants split up once more on further 

 breeding into the same proportions, while the pure domi- 

 nants and recessives each time breed true for all successive 

 generations. 



How can these Mendelian phenomena be accounted for ? 

 Mendel himself explained them in a most ingenious, and 

 yet at the same time very simple, manner. 



If of the two parent forms under consideration one parent 

 D has the dominant quality d, while the other parent R has 

 the recessive quality r, then the germ-cells of the first 

 parent will all contain the character d, those of the second 



