GENERAL ZOOLOGY 



Fig. 6.21 . The appearance of Himalayan rabbits under 

 different temperature conditions. A, a rabbit raised at 

 a temperature above 30°C. B, a rabbit raised at a 

 temperature of about 25°C. C, a rabbit which has 

 had its left flank artificially cooled at a temperature 

 below 25°C. (After R. Danneel, from R. P. Wagner 

 and H. K. Mitchell, Genetics and Metabolism, copyright 

 1955 by John Wiley and Sons, Inc., reprinted by 

 permission.) 



reared on a rich supply of moist food, the abdominal bands are almost 

 completely absent in all individuals. The same stock raised on scant, dry 

 food exhibits normal 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, and those 

 that develop later will appear normal. These flies are genetically the same; 

 the difference in their appearance is conditioned by the environment in which 

 they develop. In other words, the appearance of characters in an adult 

 individual is dependent on the presence of certain 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 individuals a character may appear in early stages of develop- 

 ment and be lacking in the adult. Age is, in this sense, a phase of the 

 environment. The relationship between genes and environment is shown 

 further by the fact that in Drosophila red color is confined to the eyes and 

 does not occur on the legs or wings. The influence of genes is likewise 

 dependent on their association with certain other genes; evidence for the 

 interaction of genes is clear-cut but very complicated and will not be given 

 here. At least 25 pairs of allelomorphic genes are concerned with eye color 



196 



