MENDELISM 123 



The same in diagram form : 



D R 



• 



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 (F^) 

 is an impure dominant. On interbreeding, the next 

 generation (Fo) 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 



