characteristics: relation to environment 247 



or by bending a leaf so as to break the conducting vessels; 

 the leaves thereupon become red in the parts with increased 

 carbohydrate. But this effect is not produced in the "green" 

 types; they do not produce the red color even in presence 

 of stored carbohydrates. 



It appears clear therefore that in maize different genes 

 cause the different varieties to have different metabolic 

 processes, different chemical reactions, some of which cause 

 I the production of colors, while others do not. Also, diverse 

 environmental conditions likewise cause changes in these 

 metabolic processes, inducing color production in some 

 cases. Whether color occurs, and its type and amount, de- 

 pend on interaction of genetic and environmental condi- 

 tions. 



Another Instructive case of the interaction of genes with 

 environmental conditions is furnished by the conditions de- 

 termining the number of facets in the compound eye of 

 Drosophila.^ In the normal eye there are present about 

 800 facets. In certain of the flies a gene at locus 57 in the 

 X-chromosome is so modified as to cause a great reduc- 

 tion in the number of fully developed facets. In this condi- 

 tion, known as Bar-eye, there are only about 80 fully de- 

 veloped facets: these form a "bar" across the eye (figures 

 9 and 10). This Bar-eye condition is dominant; the number 

 of facets is reduced even though one of the two genes of the 

 pair is normal. We may call the modified gene that causes 

 the reduction In the number of facets the Bar gene. 



The number of fully formed facets. It Is found, depends 

 on whether there Is present but one or two of the Bar 

 genes, and also on whether there Is present In addition a 

 normal gene. In the males (having of course but one X), 

 the presence of the Bar gene reduces the number of normal 

 facets to about 90. In the female. If both the X's contain 

 a Bar gene, the number of facets Is about 65. If the female 

 is heterozygotic, so that but one of her X's contains the 



