somes, but of genes, a term first used by Johannsen. If now we suppose that 

 each chromosome contains several genes, then independent assortment could 

 take place only between characteristics whose genes were in separate chromo- 

 somes. That is, two genes or determiners in the same chromosome would 

 always pass from generation to generation together — just as, in fact, the 

 "hnked characters" are found to do (see page 482). 



Chromosomes and Linkage^ The discovery of linkage as an apparent 

 exception to Mendel's rule of independent assortment of unit characters 

 turned out to be a severe test or "proof" of the theory. As experiments were 

 extended, more and more cases of linkage were discovered: not all inherited 

 traits sort out independently. Moreover, these linkages included large numbers 

 of characters, rather than two or three, which at first seemed to be exceptions 

 to Mendel's principle. 



The most telling facts came from experiments with fruit flies of the species 

 Drosophila melanogaster (see illustration opposite). Hundreds of trained work- 

 ers have studied wild forms of this species (see page 491). With the study of 

 linkages it became possible to locate the various determiners, or genes, on each 

 chromosome. One of the earliest Unkages studied in the fruit fly was the 

 case of an artificial combination containing two distinct recessive characteris- 

 tics — a very much reduced wing and a black coloration of the body (see illus- 

 tration, p. 490). 



Other examples were of linkage of this reduced wing with a certain eye- 

 color; another eye-color is usually associated with an ebony body; a vermilion 

 eye is linked with a curious notch on the wing; and so on. The linkages occur 

 in three groups of many characters and a fourth group including only a few 

 characters. These facts strengthen the suspicion (1) that each determiner 

 occurs normally in a particular chromosome, and (2) that the gene is a real 

 something, since the larger chromosomes apparently carry more determiners 

 than the small ones. 



But how can we locate a particular gene in a particular chromosome? 



Sex-Linked Characters"- The clue to identifying the chromosomes came 

 from the discovery that in many species the chromosome picture was not 



iSee No. 5, p. 504. -See No. 6, p. 505. 



488 



