244 GENETICS 



present a certain type of gene, and also a certain external 

 condition. Unless both are present, the peculiarity is not 

 produced. 



It is clear from these cases that even though an indi- 

 vidual inherits the type of gene necessary to produce a 

 certain defect, it may not be inevitable that he should have 

 that defect. A special environment may correct what in- 

 heritance leaves imperfect. These considerations apply to 

 many things in man as well as in other organisms. 



Another example of the fact that what is produced in 

 development depends both on the genes and on the environ- 

 ment is seen in the production of giants in the fruit-fly.^ 

 A modified gene, located in the X-chromosome, near its 

 left end, causes the animals (if they have no other type 

 of X-chromosomes than this) to become giants; they grow 

 to nearly twice the size of the ordinary fly. But this increase 

 in size takes place only if the animals having the modified 

 geiie are well fed during a certain period of their early 

 lives. If not well fed at this particular time in life, they 

 grow no larger than the usual flies. The giant size requires 

 to produce it a particular type of environment acting on a 

 particular type of gene. If either condition is not fulfilled, 

 giants are not produced. 



Again, in plants, the dlfi^erence between green plants, 

 containing chlorophyll, and white ones, is in some cases due 

 to a difference in genes. Plants with a certain kind of genes 

 will remain white even though grown in the light. In other 

 cases, the difference between white and green plants is not 

 the result of a difference in genes, but to a difference in 

 environment. Plants that live in the dark remain white, even 

 though they contain genes that can produce chlorophyll, 

 while similar plants grown in the light are green. That is, 

 to produce chlorophyll, a certain type of genes is neces- 

 sary, and also a certain type of environment. 



Again, there are red and white varieties of primroses; 



