238 Experimental Transformation of Species 



The Law of Segregation 



Mendel carried his experiments farther, since new ques- 

 tions grew out of his discovery of dominance. What be- 

 comes of the trait that seems to be lost in the hybrid genera- 

 tion? How do hybrids transmit their characteristics? To 

 answer these questions, Mendel crossed some of the hybrid 



p O 



Gametes Cj 1 



F:6 



O^ V--rO 



Fa 



Fig. 60. Segregation of Alternative Traits 



When two individuals showing a contrasting pair of characters (P, or parent genera- 

 tion) are crossed the offspring (Fi or first filial generation) will show the character of one 

 of the parents, the alternative remaining latent or concealed. The alternative characters 

 are represented by the light ring and the dark ring respectively. The " pure bred " in- 

 dividual can produce only one kind of germ cell or gamete, that is, the kind corresponding 

 to the particular character in question — in this case shown by the small circles, light or 

 dark. The hybrid individual (F ) can produce two kinds of gametes, correspondng to the 

 two alternative characters, in equal numbers. When a hybrid male is mated with a hybrid 

 female (F^), the two kinds of sperms can fertilize the two kinds of eggs in four different 

 combinations, resulting in three types of individuals in the next generation (Fj), namely, 

 one combining dominant gametes only, one combining recessive gametes only, and one 

 (twice as numerous as either of the others) combining a recessive with a dominant gamete. 

 Since the presence of a dominant conceals the recessive character, F2 would appear to con- 

 sist of three dominants to one recessive. This diagram applies equally to cases of incom- 

 plete dominance, such as the Andalusian Blues and Four o'clock, as to complete dominance, 

 such as Mendel described in the garden pea. 



plants with each of the parental types and others he allowed 

 to become self-fertilized. Thus, a tall hybrid was crossed 

 with a pure strain tall; and a tall hybrid was crossed with 

 a dwarf of the parental type. Similarly, plants grown from 

 the yellow seeds of hybrid origin were self-fertilized and 



