Gates: On the Nature of Mutations 



107 



This mutant originated from rubrinervis 

 in my experiments in 1907, as a domi- 

 nant heterozygous mutation, its off- 

 spring giving rubricalyx and rubrinervis 

 in a 3:1 ratio/ It is a striking color 

 variety of rubrinervis, the two being 

 morphologically identical. A rosette of 

 rubricalyx is shown in fig. 6 and a full 

 grown plant in fig. 7. In Oe. rubri- 

 calyx the under side of the leaf petioles 

 of the rosette as well as of the flower 

 stalks or hypanthia, are red, whereas in 

 rubrinervis they are invariably green.'' 



In rubricalyx there is also greater pro- 

 duction of red pigment in every part of 

 the plant, as may be observed by exam- 

 ining sections of the fresh tissues under 

 the microscope. The capacity of the 

 cells for producing anthocyanin has 

 suddenly undergone an enormous in- 

 crease. 



Any theory which will explain the 

 origin of Oe. rubricalyx will explain also 

 the origin of all Mendelian differences, 

 for this is inherited in simple Mendelian 

 fashion, wholly unlike either lata or 

 gigas. 



If we asstmie that one member of one 

 pair of chromosomes has undergone a 

 chemical change to lead to the greatly 

 increased pigment production, then we 

 can explain at the same time the origin 

 and inheritance of this mutant. We 

 therefore conclude in general that simple 

 Mendelian characters arise through an 

 alteration on the part of a chromosome, 

 analogous to the mutations which are 

 known to occur in certain bacteria. 



This type of change is of course 

 entirely different from those already 

 mentioned, and is sufficient to account 

 for the origin of all mutants which are 

 inherited in Mendelian fashion. The 

 change is fundamentally chemical rather 

 than morphological in nature, the 

 chromosomes in Oe. rubricalyx number- 

 ing 14, as in Oe. lamarckiana and Oe. 

 rubrinervis. 



Fi om such facts as these the following 

 conclusions regarding the nature of 

 mutations may be drawn. (1) Muta- 

 tions arc of many kinds and in many 

 directions. (2) Evolution is not due, as 

 Bateson** and others have urged, to the 

 loss of factors or inhibitors from the 

 germ plasm, but mutations furnish 

 abundant material for real evolution, in 

 which the modification of characters, 

 and divergences in many directions, 

 have occurred. There is really no more 

 reason for supposing evolution to have 

 resulted from "loss of inhibitors" from 

 the germ plasm than there is for the 

 embryologist to assume that the egg 

 develops into a chick by throwing off 

 inhibitors during ontogeny. This type 

 of embryological speculation was sup- 

 planted centuries ago by the observed 

 increase in complexity and structure 

 during development of the individual. 



The evolutionary conception of loss 

 of factors and inhibitors has been 

 founded upon the Mendelian presence- 

 absence hypothesis. The usefulness 

 of this symbolism in the study of the 

 inheritance of Mendelian characters 

 cannot be gainsaid. But as soon as 

 it is applied to evolutionary conceptions 

 it leads to an erroneous point of view. 

 It is safe to conclude that even apparent 

 losses, such as the origin of recessive 

 white varieties from colored varieties of 

 plants and animals, are not really due 

 to the loss of any particle from the 

 germ plasm, but to a chemical (probably 

 in some cases stereochemical) modifica- 

 tion in one element of the germ plasm, 

 viz: a chromosome or a portion of one. 



(3) Finally, another generalization 

 should now be seen to follow clearly 

 from such facts as those mentioned in 

 this paper, namely, that the inheritance 

 of any character depends to some 

 extent upon the nature of the character. 

 In other words, the manner of inherit- 

 ance of anv character is determined, or 



* Shall ''A peculiar negative correlation in Oenothera hybrids. Joiirn. Genetics 4: 83-102, pis. 

 5-6, 1914) has erroneously attempted to show that Oe. rubricalyx is not a sirnple Mendelian 

 dominant. The plants in his experiments which he treats as pure rubricalyx are obviously, from his 

 own figures, hybrids with a distinct species Oe. grandiflora, and the supposed discrepant results he 

 obtains are in fact, as far as they go, a confirmation of the writer's extensive crosses of these two 

 forms. 



" The history of the origin of this mutant is given in "vStudies on the variability and heritability 

 of pigmentation in Oenothera." Zeitschr. f. Abst. u. Vererbungslehre 4: 337-372. Figs. 5. 1 

 colored plate, 1911. 



* Problems of Genetics. London, 1913. 



