70 Experimental Zoology 



conform to expectation. One way has been to breed back the 

 first hybrids A(B) to the parent form, either A or B; the other 

 way has been to apply the rule to more than one character. 

 These two methods may now be illustrated. 



If a hybrid, A (B), is bred back to the parent type, B, half of the 

 offspring should be A(B} and half BB. This must occur be- 

 cause, on the assumption, the germ-cells of A(B) are A and B, 

 while those of BB are B and B ; thus 



If, on the other hand, A(B) is bred back to the other parent 

 type, A, all the offspring will be like A, although only half are 

 pure A's, the others being A(B}\ thus 



A B 



A A 



A A A(B) 



AA A(B) 



The most interesting test that Mendel made of his theory 

 involves the heredity of two dominant characters and two re- 

 cessive characters. Thus, if two varieties, AB and ab, are 

 crossed, the first hybrids (F-^ will be AaBb. Since A and B 

 dominate, these individuals will all resemble AB externally. 



The germ-cells of the hybrid individual, AaBb, will be of as 

 many kinds as there are possible combinations of A, a, B, b, 

 provided that each combination contains some A (or a) and 

 some B (or b), i.e. one or the other kind of the two characters, 

 Thus the only possible combinations are A B, Ab, aB, ab. 1 



1 It may seem that these four combinations do not exhaust all the possible 

 combinations of the letters, because AA, Aa, aa, BB, Bb, bb, might be sup- 

 posed to appear, but this is not the case, because on the assumption of paired 

 characters A (or a) must always be accompanied in the germ-cell with B (or 

 6) characters. Similarly, B (or 6) must always be accompanied by A (or a) 

 characters. Hence the six combinations just given are excluded. 



