68 Experimental Zoology 



assumption how this numerical relation could be explained. 

 When the male, A^ and the female, j5, germ-cells unite, every 

 cell of the hybrid will contain both A and B\ in which case 

 one dominates, namely, A, giving A{B). If we assume that in 

 the germ-cells of the hybrid the characters A and B separate 

 again, and go to different cells, half of the germ-cells will con- 

 tain the one character only, and the other half the other char- 

 acter. This is supposed to take place both in the male and in 

 the female individual. The male germ-cells containing A may 

 meet egg-cells containing A or B, and conversely the male-cells 

 B may meet egg-cells containing A or B. The possible com- 

 binations that result are shown in the following diagram : — 



I A\/B 

 i A/\B 



The chances are that, on the average, A will meet B twice as often 

 as A meets A, or that B meets B. Hence the combination A (B) 

 will occur twice as often as AA or BB. The outcome will be 

 I A A, 2 A(B), I BB. Thus according to the assumption of 

 two kinds of germ-cells in the hybrid the numerical results agree 

 with the actual results of the experiments. For this reason 

 Mendel's assumption of two kinds of gametes has been generally 

 accepted. Furthermore the theory can be tested in several 

 ways, as will be shown later, and it has so far, on the whole, 

 stood the test. When in addition to this it was found that in 

 the germ-cells a mechanism exists that seemed capable of carry- 

 ing out the postulated process of purification, it appeared to a 

 number of modern zoologists that Mendel's assumption of two 

 kinds of germ-cells in hybrids of this sort is a real and not a 

 fictitious explanation of the results. 



An actual example may make clearer Mendel's principle and 

 its interpretation. If a gray house mouse, A, is crossed with a 

 white albino mouse, B, the offspring, (Fj), will be all gray like 

 the house mouse. If these gray hybrids^ (Fj) are bred to each 



^ Following Mendel the cross between two races or varieties is called a hybrid, 

 although this term has been usually employed for crosses between species. 



