CHROMOSOMES AND MENDELIAN HEREDITY 311 



— may have a new genotypic constitution and exhibit an appearance 

 often strikingly different from that of the other branches or flowers of the 

 plant; hence the term "bud sport." Such a mutation may result in a 

 chimera, in which a distinct portion of the mature structure, commonly 

 a sharply defined sector, differs constitutionally and in appearance from 

 the other portions. Analogous changes occur in animals. Such muta- 

 tional changes are inherited only if they take place in the lineage of the 

 reproductive cells. 



Many "mutations" in both somatic and germ cells are known to be 

 due to relatively gross alterations in the chromosome complement. On 

 the other hand, the nature of the alteration in a true gene mutation is 

 unknown, since the nature of the gene itself is almost wholly a matter of 

 conjecture. The literature of genetics contains many interesting specula- 

 tions on this subject, various comparisons with molecules, radicals, 

 colloidal aggregates, and enzymes being suggested. Whatever their 

 nature, most genes appear to have a high degree of stability and are not 

 easily influenced. So far almost nothing is known about the causes of 

 natural gene mutation. Many environmental factors appear to have 

 little or no effect upon the process. Its frequency, which in nature is 

 very low, can be markedly increased by X-rays and radium and possibly 

 also by certain other agencies, such as abnormal temperature. ^° Often 

 the mutations so obtained are qualitatively the same as those occurring in 

 non-treated material. In certain cases it is apparent that the alteration 

 is a structural deficiency rather than an actual change of the gene, which 

 raises an interesting question regarding the real relation of minute defi- 

 ciencies to what have been regarded as mutations in the gene itself. 

 Whatever may be the ultimate nature of the change whose effects are 

 observed, it is probable that many variations which constitute initial 

 steps in species alteration and evolution originate in this manner.-^ 



Assuming, as many do, that genes are discrete material particles, 

 tentative calculations place them well below the limit of visibility with 

 the ordinary microscope and suggest that they are of the same order of 

 magnitude as certain large protein molecules. ^^ No worker should claim, 

 however, that any accurate determination of their size is at present 



20 X-rays: Muller (1927, 1928a6, 1929c, 1930a6), Weinstein (1928), Whiting (1928), 

 Stadler (1928a6c, 1929, 193Ca6c, 19316, 1932), Hanson (1928), Patterson (1928, 1929), 

 Gowen (19296), Goodspeed and Olson (1928a6), Goodspeed (1930a6c), Oliver 

 (1929), Patterson and Muller (1930), Whiting (19326) and Timof eeff-Ressovsky (1930). 

 Earth radiation: Olson and Lewis (1928) and Hanson and Heys (1929). Radium: 

 Gager and Blakeslee (1927), Stadler (19286), Goodspeed (1929), Hanson and Heys 

 (1928), Hanson and Winkleman (1929), and Buchholz and Blakeslee (19306). Tem- 

 perature: Muller and Altenburg (1919), Muller (1928a), Goldschmidt (1929a), Jollos 

 (1930), Rokizky (1930), and Plough and Ives (1932). 



21 See Conklin (1919-1920), Muller (1922, 1923), Muller and Altenburg (1919), 

 Morgan (1922), Castle (1923), Hagedoorn et al. (1921), and Gates (1915, 19206). 



22 See Morgan (1922) and Gowen and Gay (1933). 



