GENETICS AND EUGENICS 287 



peas, one-third of them (constituting one-fourth of the whole num- 

 ber) produced tall peas without deviation, but the other two-thirds 

 (half of the whole number) produced stock that repeated the produc- 

 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 mernhers 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 will show 

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

 ollaries 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 (which we shall call an allelomorph) 

 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 determined only after a cross 

 has been made, and the determination holds only for the species ob- 

 served. Thus, tallness may be dominant over dwarfness in one species 

 of plant and recessive in another. In many cases, dominance is not 

 complete, and in a few classic 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 



