LINKAGE, CROSSING-OVER, AND THE GERM PLASM 287 



then, than to assign these two large linkage groups to the two large 

 chromosomes that are present? 



All went well with the linkage hypothesis for awhile, but before 

 long one of the workers discovered a new character that was not at 

 all linked with any of the three groups and therefore could not be 

 assigned to any chromosome known at that time. This seemed at 

 first like a staggering blow to the hypothesis then entertained, but it 

 turned out to be one of the best demonstrations of its validity. A re- 

 examination was made of the germ cells of Drosophila with the result 

 that a pair of tiny chromosomes was found to be always present, which, 

 because of their very small size, had been overlooked by the original 

 students of this material. This tiny chromosome was called Chromo- 

 some IV, and the new mutant, bent wing, was assigned to it. Some 

 time later another aberrant mutant, eyeless, was found that was closely 

 linked with bent, and therefore assigned to Chromosome IV. So far, 

 these are the only genes that have been located in the tiny chromosome. 

 This may mean that there is not room for many genes in so small a 

 body. 



Now, if there is anything in the chromosome theory of heredity, 

 and if the genes of individual differences have their seat in the chromo- 

 somes, all of the character differences in Drosophila melanogaster, no 

 matter how many are found, must be in no more than four groups, for 

 there are only four kinds of chromosomes in that species. For 

 a while it was feared that some new character would appear that was 

 not linked with any of the known linkage groups, for the discovery of 

 such a character would strike a severe blow against the theory of link- 

 age and against the chromosome theory in general. After the passage 

 of several years, however, and the discovery of almost a hundred new 

 mutants, not one has been found that does not show linkage with one 

 of the four known groups. Just to the extent that the finding of a 

 fifth group of characters would have weakened the chromosome theory, 

 to that extent the failure to find any exceptions to the four linkage 

 groups strengthens the theory. 



The characters represented by genes in both second and third 

 chromosomes have to do with all parts of the body, including eye 

 color, eye shape, body color, wing size and shape, bristle characters, 

 leg form, and lethal characters. It cannot then be said that any one 

 chromosome carries genes characteristic of any one part of the body; 

 instead, it seems that every chromosome carries genes that affect every 

 part of the body or, in other words, the whole organism. 



