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 mdeperident 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 dwarfness is its alternative recessive. When a 

 dwarf appears, therefore, there is present no dominant tallness to 

 suppress it. In the Fi generation all the individuals were tall because, 

 although they had all received the recessive character of dwarfness 

 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 19 18). 



