CHAPTER XV 

 MENDELISM AND MUTATION 



MENDELISM 



The classic researches carried out by Mendel a half -century ago on 

 the hybridization of garden peas are now so well known that a detailed 

 description of them would be superfluous here. Moreover, since the 

 main principles of Mendelism are illustrated in the results of the simplest of 

 Mendel's experiments, a review of one or two of the latter will for our 

 purposes be sufficient. 1 



A Typical Case of Mendelian Inheritance. — Mendel crossed plants of a 

 pure bred race of tall peas (6 to 7 feet in height) with plants of a pure 

 bred dwarf race (% to \y 2 feet in height) (Fig. 129). All the plants 

 of the first hybrid generation (Fi) were tall like -one of their parents. 

 When these tall hybrids were self-fertilized or bred to one another, it 

 was found that the second hybrid generation (Fz) comprised individuals 

 of the two grandparental types, tall and dwarf, in the relative numerical 

 proportion of 3:1. It was further found that the tall individuals of this 

 generation, though alike invisible characters, were unlike in genetic con- 

 stitution: one-third of them, if bred for another generation, produced 

 nothing but tall offspring, showing that they were "pure' : for the 

 character of tallness; whereas the other two-thirds, if similarly bred, 

 produced again in the next generation both tall and dwarf plants in the 

 proportion of 3 : 1, showing that they were hybrids with respect to tallness 

 and dwarfness. The dwarf plants of the second hybrid generation (F 2 ) 

 produced nothing but dwarfs when interbred; they were "pure ,; for 

 dwarfness. From these facts it was evident that the plants of the F 2 

 generation, although they formed only two visibly distinct classes, were 

 in reality of three kinds: pure tall individuals, tall hybrids, and pure 

 dwarfs, in the relative numerical proportions of 1 :2:1. 



The explanation offered by Mendel for these phenomena may be 

 briefly stated as follows (Fig. 129). The germ cells produced by the 

 pure tall plant carry something (now termed a factor, represented here 



1 Detailed accounts of the many facts of Mendelism may be found in more special 

 works on the subject. See Morgan et at. 1915, Chapters 1 and 2; Bateson 1913; 

 Castle, Coulter et al. 1912; Castle 1916; Coulter and Coulter 1918; Babcock and 

 Clausen 1918, Chapter 5; Punnet 1919; Darbishire 1911; Morgan 1919a; Thomson 

 191.3; East and Jones 1919. 



336 



