458 THE MAINTENANCE OF SPECIES 



a good deal about their behavior, although the scholarly attack upon 

 the gene in the light of what is sure to follow can be said to have 

 hardly begun. 



It is plain that there are many more distinguishable traits and 

 characters present in an organism than there are chromosomes. In 

 Drosophila, for example, which has only four pairs of chromosomes to 

 a cell, over five hundred hereditary differences have been accurately 

 identified. Consequently, many determining genes must be located 

 in each pair of chromosomes. What has been found to be true of 

 Drosophila in this respect, is undoubtedly true of other organisms. 

 So much of our knowledge of genes in general has been acquired by 

 investigations upon the ubiquitous banana-fly that genetics stands in 

 some danger of becoming Drosophiletics. These tiny flies, that have 

 never even heard of birth control, lend themselves very favorably to 

 the study of genes. Within a month a single pair can become grand- 

 parents of so many grandchildren that it is difficult to keep track of 

 them. Millions have actually been experimentally bred and critically 

 examined one hy one by different workers within the past three decades 

 since their scientific usefulness has been discovered. They even 

 gained the Nobel prize award (1934), with the aid of Dr. Thomas 

 Hunt Morgan and his associates. 



It has not only been possible for the investigators of Drosophila 

 to determine more than five hundred determining genes in these flies, 

 but also even to locate these several genes definitely in particular 

 pairs of chromosomes, and to arrange them with reference to each 

 other at definite distances apart within a single chromosome. All 

 that has been learned by the followers of Mendel about the interaction 

 of what are termed "factors" appUes to the invisible genes. For 

 example, it is not likely that single genes, any more than single 

 factors, "determine" single somatic traits or characteristics. Rather 

 the genes must work together to bring about visible results, since 

 "genie balance" is essential to somatic success. 



Linkage and Crossing-over 



Although there is, as Mendel demonstrated, independent assortment 

 between different chromosomes during the formation of the gametes, 

 the genes that are located in any single chromosome tend to hang 

 together in succeeding generations and not to become separated from 

 each other. This is called linkage. By means of it, whole blocks of 

 genes may act together as a unit in heredity. 



