MENDEL'S LAWS OF INHERITANCE 281 



It seemed evident that some basic principle was common to all these 

 crosses, and Mendel now sought to discover what this basic principle 

 might be. He assumed that each individual must have two factors for 

 each quality but that a germ cell produced by the individual could carry 

 but one factor from any pair present in the individual. It would follow, 

 then, that factors would be separated at germ-cell formation and united 

 into pairs at fertilization. 



We can diagram this idea, using the cross between the tall and dwarf 

 peas, by letting the capital letter D represent the factor for tallness and 

 the small letter d represent the factor for dwarfness (which, as we have 

 seen, acts as an alternative to tallness in inheritance in peas). (Mendel's 

 original notation is here slightly changed to make it conform to modern 

 usage.) 



The following diagram shows that Mendel's hypothesis fits all the 

 observed facts: 



Diagram of Monohybrid Cross 



The original parent generation Pi Tall X dwarf 



Gene formula of Pi DD dd 



Formulas of gametes produced by each parent . D d 



Resulting formula of Pi generation Dd 



(All tall, all alike) 

 Gametes produced by each Pi individual (two 



kinds in equal numbers) D and d 



Union of gametes to produce P 2 zygotes, when I Female gametes D d 



Pi is self-fertilized or interbred ] Male gametes D d 



~DD Dd 

 Probable proportion of all possible com- dD dd 



binations DD (2)Dd dd 



The P 2 generation 1 tall (DD)— 2 tall (Dd)—\ dwarf (dd) 



3 tall 1 dwarf 



1. Each of the original parents (Pi generation) has both factors alike, 

 DD or dd, because it came from a variety proved to breed true for its 

 respective quality. 



2. As a consequence, any germ cell produced by the tall parent could 

 only receive a D, and any produced by the dwarf parent could only 

 receive a d. 



3. The cross must bring together a gamete with D and a gamete with 

 d, and the resulting zygote that develops into an i*\ individual must have 

 the formula Dd. (Since the individuals of the F h with the formula Dd, 

 are as tall as the original tall parent with the formula DD, D is said to be 

 dominant to d, and tall is said to be dominant to dwarf.) 



4. When an F\ individual produces its germ cells, there will be two 

 kinds in equal numbers, D and d. This will apply to the formation of both 

 male and female gametes. 



