386 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
_ MENDEL’S EXPLANATIONS' 
JOHN M. COULTER AND MERLE C. COULTER 
Mendel’s explanation of this behavior involved three theses which 
at that time were new to biology. These theses must be kept distinct 
from one another. 
1. Independent unit characters.—This means that an organism, 
although representing a morphological and physiological unity, from 
the standpoint of heredity is a complex of a large number of independ- 
ent heritable units. Thus if one pea plant is tall and another one is 
dwarf the behavior of the hybrid produced from them with reference 
to this character will be the same, no matter what other characters 
the parent plants may have had. In other words, the characters 
are independent units, unaffected by other characters or units. The 
character of tallness from a tall plant with wrinkled seeds or purple 
flowers will act just the same as from a tall plant with smooth seeds 
or white flowers. Tallness is a unit and its behavior in inheritance is 
independent of all other units. 
2. Dominance.—In the germ plasm there are certain determiners 
of unit characters which dominate during the development of the 
body, causing these characters to dominate over others and thus 
become visible. The characters dominated over and thus not allowed 
to express themselves are called recessive characters. ‘These recessive 
characters are present in the germ plasm, but cannot express them- 
selves and become visible as long as the dominant characters are pres- 
ent. When a dominant character is absent, however, its recessive 
alternate is free to express itself and become visible. 
For example, in the case of tall and dwarf peas, tallness is a domi- 
nant character and dwariness is its alternative recessive. When a 
dwarf appears, therefore, there is present no dominant tallness to 
suppress it. In the F, generation all the individuals were tall because, 
although they had all received the recessive character of dwatfness 
from one of the parents, they had received the dominant character 
of tallness from the other parent, and so dwarfness did not appear in 
any of them. Such pairs of alternative characters are now commonly 
called allelomorphs. Thus tallness and dwarfness are allelomorphs 
in the pea, one dominant over the other, which is therefore recessive. 
3. Purity of gametes.—A gamete can contain only one of two 
alternative characters. For example, it may contain the character 
* From Coulter and Coulter, Plant Genetics (The University of Chicago Press 
copyright 1918). 
