GENETICS: THE HERITABLE TRANSMISSION OF CATALYSTS 167 



AAAXX Intersex, sterile 



AAAXXY Intersex, sterile 



AAAXY "Supermale," sterile 



It must not be supposed, however, that heredity is carried by 

 the genes alone, or even solely by the chromosomes in which x-ray 

 treatment may produce "position effects" having consequences 

 as marked as "point mutation." In Chapter 9, these matters are 

 discussed at length in connection with differentiation and morpho- 

 genesis. However, we may here refer to a few other non-genic 

 and non-chromosomal factors. 



First are the so-called "maternal effects," observed for example in 

 the mode of coiling of the shell of the snail Limnaea peregra; the 

 direction of coiling (righthand or lefthand) depends in the F x gener- 

 ation upon the genotype of the mother, not upon the genotype of the 

 individual (Sturtevant, 1923). The work of Toyama (1912) with silk- 

 worms (Bombyx mori) indicated that the appearance of an egg (shape, 

 color) depends not upon its own genetic constitution (maternal and 

 parental genes), but upon the genes of the mother in whose body the 

 egg developed. 



Definite cytoplasmic inheritance is illustrated by the self-reproduc- 

 ing plastids, e.g., chloroplasts which largely dominate photosynthesis 

 in plant cells. 5 Wanda K. Farr has shown 6 that cellulose is formed 

 in a chloroplast or colorless plastid. Starch granules likewise develop 

 by deposition of material from the plasma of certain plastids around 

 small starch-forming centers. 



In the case of variegated plants, the leaves show whitish spots or 

 stripes where the cells are devoid of chlorophyll generally because 

 viruses (infective agents approaching molecular dimensions) destroy 

 the chlorophyll or prevent its formation. These viruses, and there- 

 fore their effects, may be heritably transmitted; variegation apparently 

 does not harm the plant and is regarded as a desirable feature by 

 nurserymen. On the other hand, some viruses produce very undesir- 

 able effects, e.g., influenza, poliomyelitis, foot-and-mouth disease, yellow 

 fever, chicken sarcoma (Rous), rabbit papilloma (Shope), encephalitis. 

 But though these may be transmitted by some form of infection or 

 inoculation, and may lead to formation of specific antibodies, they 

 are not effectively transmitted by heredity. However, we do not know 

 to what an extent harmless or beneficial virus-like symbionts or 

 "diseases" may be carried by heredity, as is the latent potato mosaic 

 virus which shows its effects only in susceptible strains of potato. 



But in many plants plastid color is gene-determined, and is trans- 

 mitted on a Mendelian basis. 7 Most geneticists express the Scotch 

 verdict of "not proven" on experimental work supposed to demon- 



