HYBRIDIZING PLANTS 127 



a further test of his theory was made. It was found in Fa 

 that about 1/3 of the tall F2 plants bred true and 2/3 behaved 

 like the Fi plants, producing a 3 : 1 ratio. All of the F2 dwarf 

 plants bred true. There is still another way by which the 

 theory could be tested. If the Fi were to be backcrossed to 

 the parents, we would expect different results. If the Fi were 

 backcrossed to the tall parent (Tt X TT) , we would expect 

 all the progeny to be tall since all of them would contain at 

 least one T. But if the Fi were backcrossed to the dwarf 

 parent (Tt X tt) , we would expect one half of the progeny 

 to be tall (Tt) and one half dwarf (tt). Again Mendel saw 

 the possibility, and the final test of his theory was made. 

 The results were exactly as expected. The backcross with 

 the dwarf parent gave a ratio of 1 dominant to 1 recessive, 

 while the backcross to the dominant parent yielded only 

 dominant progeny. Thus, the first Mendelian law of inheri- 

 tance was discovered before anyone had ever described a 

 chromosome or heard of a gene. The law can be stated as 



follows: THE MEMBERS OF PAIRS OF ALTERNATE FACTORS OR 

 ELEMENTS SEPARATE INTO SISTER GAMETES. This is kuOWU 



as the law of segregation or Mendel's first law. 



Mendel did not stop here but proceeded methodically 

 toward the development of his second law. The next step 

 was to cross two strains which differed in two ways. Thus, 

 his eighth experiment was to cross a variety of peas with 

 round, yellow seeds with a variety having wrinkled, green 

 seeds. As we might expect, the Fi consisted of plants with 

 round, yellow seeds, since round is dominant to wrinkled and 

 yellow dominant to green. His results for the F2 are as 

 follows : 



315 round yellow (parental combination) 



101 wrinkled yellow (recombination) 



108 round green (recombination) . 



32 wrinkled green (parental combination) 



556 



The first point to note is that while the original parents were 

 round yellow and wrinkled green, the F2 has two new classes. 



