GENETICS AND EUGENICS S23 



tion of tall and dwarf peas in the same proportion as did the preced- 

 ing generation. Here, then, was a definite ratio of 1:2:1. Mendel 

 tested his findings with six other traits of the pea and with more than 

 a dozen other kinds of plants, and after verifying his results he was 

 able to announce the following law : When members of a species hav- 

 ing contrasting characters are crossed, all the immediate offspring 

 will show the trait that is dominant, hut if the members of this gen- 

 eration are bred to themselves, one-fourth of the offspring ivill show 

 the dominant trait and breed true for it; one-fourth will show the 

 recessive trait and breed true for it; the remaining two-fourths of 

 the offspring will show the dominant trait, but will reproduce the 

 contrasting characters in the same proportion as did their immediate 

 parents. 



Derivatives of Mendel's Law 



From the fundamental law which Mendel discovered, several corol- 

 laries can be drawn, based on factors responsible for the distribu- 

 tional behavior of inheritable characters. These corollaries are : 



Principle of Dominance. — The determiner for one member of a 

 pair of contrasting characters (pairs of contrasting characters are 

 called allelomorphs) may take precedence over the other member 

 when the two have been brought together in a cross between unlike 

 parents. Which trait is dominant and which is recessive can be de- 

 termined only after a cross has been made, and the determination 

 holds only for the species observed. Thus, taUness may be dominant 

 over dwarfness in one species of plant and recessive in another. In 

 many cases, dominance is not complete, and in some instances it is 

 lacking altogether. These cases will be discussed later in this 

 chapter. 



Independence of Unit Characters. — The determiner for any given 

 character acts as an independent hereditary unit as it is passed along 

 from generation to generation. In each generation it may have a 

 different set of associates, but its associations in one generation do not 

 affect the company it may keep in the next. Thus tallness in peas may 

 be associated with yellow pod and with wrinkled seed coat in one 

 generation, but it may form a combination with green pod and smooth 

 seed coat in the next. This important principle is responsible for the 

 great variation we find in plants and animals and makes it possible 

 for a breeder to bring about any desired combinations of the traits 



