CONTINUITY OF THE RACE 



595 



esses, though we see its ultimate effect 

 only in eye color. In experiments with 

 Drosophila where the location of the gene 

 in the giant chromosomes of the salivary 

 gland cells can actually be seen through the 

 microscope, it is observed that when a piece 

 of the chromosome is missing which con- 

 tains the gene for eye color, progeny from 

 such homozygous individuals are either 



much weakened or do not survive at all. In 

 other words, the gene that ultimately regis- 

 ters itself in eye color plays other roles 

 along the way that are vital to the adult 



organism. 



An interesting example of this multiple 

 effect of single genes in humans is illus- 

 trated by the rare anomaly known as 

 phenylketonuria. In this defect, a simple 



Fig. 24-9- Blending dominance can be illustrated with red and white cattle. Here the genotype can 



be distinguished by simply observing the coat color. 



