272 FOUNDATIONS OF BIOLOGY 



whom they would have meant so much in his work to secure 

 experimental data in heredity. To-day Mendelism is 

 essentially a science in itself, with its own vocabulary of 

 technical terms. We can attempt no more than to make 

 clear its fundamental features by a few concrete examples; 

 the first from Mendel's own work. 



Mendel chose seven pairs of contrasting, or alternative, 

 characters which he found were constant in certain varieties 

 of edible Peas, such as the form and color of the seeds, whether 

 round or wrinkled, yellow or green; and the length of the 

 stem, whether dwarf or tall: and these he studied in the 

 HYBRIDS. One ordinarily thinks of a hybrid as a cross be- 

 tween two species or, at least, two characteristically distinct 

 varieties of animals or plants; but as a matter of fact the off- 

 spring of all sexually reproducing organisms are really hybrids 

 because two parents seldom, if ever, are exactly the same in 

 all of their germinal characters. Consequently the offspring 

 are hybrids with respect to the characters in which the par- 

 ents differ. 



1. Monohybrids 



Mendel found, for example, in the cross between the tall 

 and dwarf varieties of Peas, that all of the progeny in the 

 FIRST FILIAL (Fi) generation were tall like one parent, there 

 being no visible evidence of their actual hybrid character. 

 Accordingly tallness'was designated a DOMINANT (D) and 

 dwarfness a RECESSIVE (d) character. His next step was to 

 follow the behavior of these characters in succeeding genera- 

 tions. Therefore the tall hybrids (Fi) were inbred (self-fer- 

 tilized) and their offspring, the SECOND FILIAL (F 2 ) generation, 

 were found to be tall and dwarf in the proportion of three to 

 one (3D :ld) . This is now the broadly established MENDELIAN 

 RATIO. Of course in dealing with a small number of individ- 



