yo Experimental Zoology 



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

 first hybrids A{B) to the parent form, either A ox 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 — 



A B 



B B 



A{B) BB 



A(B) BB 



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

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

 pure ^'s, the others being A(B); thus — 



A B 



A A 



A A A(B) 



A A 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 (Fj) 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 Ay 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 AB, Ab, aB, ab.^ 



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

 combinations of the letters, because A A, 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 

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

 characters. Hence the six combinations just given are excluded. 



