Genetic Factors 425 



The evidence therefore favors the view that in most cases, certainly, 

 genes control form directly. How this is done poses one of the most 

 difficult problems of genetics and morphogenesis and one closely in- 

 volved with that of biological organization itself. Most of the work on 

 gene action has indicated that genes control the production of specific 

 substances; but how, one may ask, does a substance control the de- 

 velopment of a specific form? This involves the control of relationships, 

 like that between length and width in a developing fruit, and not only 

 single relationships but a whole series of them organized into a pattern. 

 The amount of growth in one dimension is related precisely to the 

 amounts in all the others. For a specific substance to do this involves the 

 old question of organizers and organ-forming substances which are so 

 easy to postulate and so difficult to picture in physical terms. These 

 gene-produced substances seem rather to act as evocators, calling out or 

 modifying formative potentialities in the living stuff of the organism. We 

 need to assume the immanent presence in protoplasm of something that, 

 for want of a better simile, can be compared to a system of coordinates in 

 three dimensions. 



A specific substance may be thought of as bending or otherwise modify- 

 ing these coordinates in a particular way and thus regulating growth in 

 such a fashion that a specific pattern is produced. The problem remains 

 as to the nature of this underlying formativeness. It may be thought of, 

 perhaps, as a molecular pattern in the cytoplasm (p. 455). To call it a 

 "field" is to give it a name but not an explanation. It is evidently involved 

 in the nature of the living, organized system that an organism is. To 

 recognize that it exists, even though one cannot yet explain it, is a step 

 in advance and may save us from a too naive conception of the nature 

 of gene action in development. 



Other Structural Traits. Aside from pure forms in the geometrical sense, 

 various other structural characters are under gene control, or at least 

 show mendelian segregation after crossing. Thus the zigzag stem that 

 appeared in descendants of a certain cross between Tom Thumb pop- 

 corn and Missouri dent (Eyster, 1922) behaves as a double recessive to 

 normal stem. "Corn grass," a mutation in maize with narrow leaves, 

 many tillers, and few male flowers, is a single-gene dominant ( Singleton, 

 1951 ) . In peas, relatively long distance between first and second flowers as 

 compared with the total length of the inflorescence was found by 

 Lamprecht ( 1949 ) to be the result of three genes. Abnormalities of 

 various sorts have also been shown to have a genetic basis. Among them 

 are double flowers in many plants, as for example Tropaeolum (Eyster 

 and Burpee, 1936), where doubleness is recessive but where a dominant 

 "super-double" strain appeared, female sterile and with about 135 

 petals. Peloria in Linaria vulgaris and Digitalis purpurea has been shown 



