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 1% 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 (F 2 ) 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 in visible 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-z) 

 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 el al. 1915, Chapters 1 and 2; Bateson 1913; 

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

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

 1913; East and Jones 1919. 



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