castle. — mendel's law op heredity. 



541 



stable) forms from such as are similar to them in appearance but still 

 hybrid iu one or more characters. Mendel has generalized these state- 

 ments substantially as follows: In cases of complete dominance, when 

 the number of differences between the parents is ?i, the number of 

 different classes into which the second generation of offspring fall will 

 be 3", of which 2 n will be pure (stable); the remainder will be hybrid, 

 though indistinguishable from pure individuals. The smallest number 

 of individuals which in the second hybrid generation will allow of one 

 pure individual to each visibly different class, will be 4". See Table IV. 



TABLE IV. 



The law of Mendel reduces to an exact science the art of breeding in 

 the case most carefully studied by him, that of entire dominance. It 

 gives to the breeder a new conception of " purity." No animal or plant 

 is " pure " simply because it is descended from a long line of ancestors 

 possessing a desired combination of characters ; but any animal or plant 

 is pure if it produces gametes of only one sort, even though its grand- 

 parents may among themselves have possessed opposite characters. The 

 existence of purity can be established with certainty only by suitable 

 breeding tests (especially by crossing with recessives), but it may be safely 

 assumed for any animal or plant descended from parents which were like 

 each other and had been shown by breeding tests to be pure. 



Special cases under the law of Mendel. It remains to speak of some 

 special cases under the law of Mendel, which apparently are exceptions 

 to one or another of the principles already stated, and which probably 

 result from exceptional conditions known to us only in part. These 

 special cases have come to light iu part through Mendel's own work, in 

 part through that of others. 



