30 Plant Genetics 



and physiological unity, from the standpoint of heredity 

 is a complex of a large number of independent 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 independ- 

 ent 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 themselves and become visible as 

 long as the dominant characters are present. When a 

 dominant character is absent, however, its recessive alter- 

 nate is free to express itself and become visible. 



For example, in the case of tall and dwarf peas, tall- 

 ness is a dominant character and dwarfness is its alter- 

 native 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 



