THE MOUSE 429 



holds true not only for the inheritance of the color of mice but 

 of most other animals. Indeed, many other characteristics 

 of both plants and animals are inherited in these same propor- 

 tions. The reason for this law is as follows : when the parents 

 differ greatly in that one has and the other lacks a character- 

 istic, the differences in the parents do not blend in the offspring, 

 but all of the offspring show the characteristic. This pres- 

 ence of a characteristic is said to be " dominant " over its 

 absence. The absence of the character — the recessive condi- 

 tion — is, however, not lost but exists in the reproductive cells 

 of the offspring ; half of the reproductive cells possess the 

 characteristic and half lack it. Now if two individuals with 

 an equal number of germ cells of the dominant and the recessive 

 types are crossed, their germ cells meet in pairs, and it can be 

 shown that, in the long run, the following four combinations 

 of the two types are equally apt to occur. In the following 

 formula the letter d stands for the dominant condition, the 

 letter r for the recessive, the sign i for the germ cells from 

 the male parent, and the sign 5 for the germ cells from the 



female parent. 



$ d 9 d, i d 9 r, $ r 9 d, t r 9 r. 



\Vhenever the combination contains the dominant character, 

 the body of the fully grown offspring will show only the domi- 

 nant character. It is easy to see that three-fourths of the 

 offspring will be dominant, and one-fourth recessive. The fore- 

 going law and its explanation were discovered forty years ago 

 by a monk, Gregor Mendel, in his monastery garden in Aus- 

 tria.' The law is important because it enables us to prechct 

 what proportion of the grandchildren will show the domi- 

 nant character of their grandparents or its absence. Men- 



1 Fig. 39.3, 



