CASTLE. — MENDEL S LAW OF HEREDITY. 



537 



B(A). But whatever the character of the hybrid may be, its germ-cells, 

 when mature, will bear either the character A or the character B, but not 

 both ; aud As and Bs will be produced in equal numbers. This perfectly 

 simple principle is known as the law of " segregation,^' or the law of the 

 " purity of the germ-cells." It bids fair to prove as fundamental to a right 

 understanding of the facts of heredity as is the law of definite proportions 

 in chemistry. From it follow many important consequences. 



A first consequence of the law of purity of the germ-cells is polymor- 

 phism of the second and later hybrid generations. The individuals of 

 the first hybrid generation are all of one type, provided the parent races 

 are pure. Each has a character resulting from the combination of an 

 A with a B, let us say AB. [In cases of dominance it would more 

 properly be expressed by A (B) or B (A).] But in the next generation 

 three sorts of combination are possible, since each parent will furnish 

 As and Bs in equal numbers. The possible combinations are AA, AB, 



TABLE I. 



and BB. The first sort will consist of pure As and will breed true to 

 that character ever afterward, unless crossed with individuals having a 

 different character. Similarly the third sort will be pure Bs and will 

 breed true to that character. But the second sort, AB, will consist of 

 hybrid individuals, like those of which the first hybrid generation con- 

 sisted. If, as suppo.'^ed, germ-cells, A and B, are produced in equal 

 numbers by hybrids of both sexes, and unite at random, combinations 

 AA, AB, and BB should occur in the frequencies, 1:2:1. For in 

 unions between two sets of gamete-^, eacii A -|- B, there is one chance 

 each for the combinations A A and BB, but two chances for the com- 

 bination AB. 



