Experimental Hybridizing 



69 



other, their offspring {F^ will be gray or white in the proportion 

 of 3:1. If the white individuals are inbred, they will give 

 only white, and this is true for all of their descendants. They 

 are said therefore to be "pure." The gray individuals, how- 

 ever, show themselves to be of two kinds; one third of them, 

 if inbred, produce only gray, and all of their descendants will 

 be gray. They, too, are said to be "pure." The other two 

 thirds, if inbred, produce both white and gray mice. If these 

 offspring are examined by further crossing, it is found that the 

 whites are pure and give only whites; that some of the grays 

 are "pure" gray, but the others are gray-dominant- white- 

 recessives, A{B)^ and again in these we find the proportion i A : 

 2 AB : 1 B. The following scheme will show at a glance 

 the succession of generations : — 



A practical consideration of some importance is obvious from 

 these results. Pure races can be obtained from the hybrid, 

 A(B), by selecting the offspring with "pure" germ-cells, A or B. 

 On the other hand, the A(B) hybrids always produce some 

 A(B), so that all their offspring do not return entirely to the 

 two parental types, but in every succeeding generation they will 

 continue to split off some "pure" ^'s and B's. 



As has been stated, Mendel's assumption in regard to the two 

 kinds of germ-cells has been tested in other ways and found to 



