MENDEL'S EXPLANATION 183 



(J gametes 

 D R 



R 



Of 



R 



This type of mating is of great practical value 

 because it is tlie one by means of which the question 

 whether an animal or plant bearing the dominant 

 character is heterozygous or homozygous can be 

 most expeditiously decided. This question could, 

 it is true, be answered by mating the individual 

 in question with another hybrid, if a hybrid of the 

 first generation were at hand (a hybrid of the second 

 generation can only be known to be such by testing 

 it, and that is what we are trying to do) ; and even 

 if this were available an inconveniently large number 

 of offspring would have to be produced before there 

 were sufficient grounds (derived from this test) for 

 believing that the individual tested was not a hybrid. 

 Directly a recessive is produced by mating a par- 

 ticular individual, which may be called A for brevity, 

 with a hybrid, it is certain that A is a hybrid ; but 

 the production of a family of six containing no 

 recessives does not by any means prove that A is 

 a dominant ; the next child might be a recessive. 

 But the probability of A being a dominant becomes 



