INHERITANCE 313 



Mendel's law assumes that the gametes bearing the 

 characters of the two parents are produced in equal num- 

 bers, and distributed at pairing in accordance with the law 

 of chance; and this assumption is not contradicted by the 

 known facts. And since it offers a simple mathematical 

 basis for calculating the results of a variety of crosses, it may 

 readily be tested, as, for example, by backcrossing a hybrid 

 with an individual of either parent stock. If mated with 

 the recessive stock, the result should be as follows (given, of 

 course, as for any test, a sufficient number of offspring) : 



DandR, germ cells of the hybrid parent 

 I y^ I in chance combination with 

 RandR, germ cells of the recessive parent, 

 give in the second (F J generation. 



2D (R)* + 2RR i. e., 50% of each color. 



In the more typical cases of alternative inheritance, all 

 the foregoing proportions have been substantially realized 

 in breeding experiments. 



When the parents differ in two or more characters, the 

 hybrids bearing germ cells that bear all these characters 

 severally, will effect new combinations of them, and forms 

 differing from either parent will appear in the second and 

 later generations. If we let X and Y represent the dominant 

 and X and y the recessive phase of two characters (as, for 

 example, eye color and hair length) there will appear in the 

 second generation, besides the unstable hybrid forms, the 

 stable forms XXYY, XXyy, xxYY, and xxyy. Which- 

 ever two of these represent the combination of characters 

 found in the parents, the other two are new combinations. 

 The law of Mendelian inheritance, substantially as estab- 



*The parenthesis is thus used as a convention for indicating the 

 recessive character. 



