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 ahybrid, 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 like A, although only half are 
pure A’s, the others being A(B); thus — 
A B 
A A 
AA 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, 8, 
provided that each combination contains some A (or a) and 
some B (or 6), 7.e. one or the other kind of the two characters, 
Thus the only possible combinations are AB, Ab, aB, ab.’ 
"Tt 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 
b) characters. Similarly, B (or 6) must always be accompanied by A (or a) 
characters. Hence the six combinations just given are excluded. 
