484 GENETICS 



certain combination of genes always gives a certain type of indi- 

 vidual. However, if the environment be changed and the genetic 

 combination remain the same, the individuals may be different 

 (c/. p. 439). For example, the red primrose has red flowers if kept 

 at a temperature ranging from 15° to 20° C. A plant with the 

 same genes but reared at a temperature of 30° to 35° C, with other 

 environmental conditions unchanged, produces white flowers. If a 

 plant with white flowers is brought into a room at 15° to 20° C, the 

 flowers that develop later will be red. The effect of the genes for 

 color of flower is limited by the temperature of the environment in 

 which the flowers develop. That the gene is not altered is shown 

 by the ability of the plant to cease producing white flowers and give 

 rise to red ones at a different temperature. 



The same type of effect has been demonstrated in Drosophila. 

 A certain race of the fruit-fly is distinguished by the fact that there 

 are almost no black bands on the abdomen in contrast to the regular 

 bands of the normal race. When this race is reared in a rich supply 

 of moist food, the abdominal bands are almost completely absent in 

 all individuals. The same stock, raised on scant, dry food, appears 

 normal as to banding of the abdomen. If a culture is started with 

 abundant moist food which is not replenished but allowed to 

 become dry, the individuals that develop first will show abnormal 

 banding, while those that develop later will appear normal. These 

 flies are genetically the same, but their appearance is determined 

 by the environment in which they develop. In other words, the 

 appearance of characters in an adult individual is dependent on the 

 presence of genes in the zygote, acting in a particular environment 

 during development. 



The age of an individual sometimes affects the development of 

 characters. Certain characters may not appear until the individual 

 reaches a particular age. In other cases, a character may appear 

 in early stages of development and be lacking in the adult. Age 

 is, in this sense, a phase of the environment. 



Another condition that shows the relationship between genes 

 and environment is the fact that the genes that determine red color 

 in eyes do not cause this color on legs or wings. The mechanism 

 of mitosis makes it reasonable to suppose that all genes are equally 

 distributed to all cells. Yet these genes are effective in determin- 

 ing an external character only in certain parts of the organism. 



The influence of genes is likewise dependent upon their associa- 



