196 



THE AMERICAN NATURALIST 



[Vol. L 



Recent work on Drosophila has borne out in a striking 

 way the conclusion that the separation of factors just dis- 

 cussed is due to chromosomal interchange. It will he re- 

 membered that the pairs of factors in the example under- 

 went recombination in only about 42 per cent, of the eggs, 

 i. e., they held together more often than they separated, 

 and so might be said to be partially linked. Their mode 

 of inheritance therefore forms a contrast not only to com- 

 plete linkage, but also to the familiar cases of random 

 assortment, where two pairs of factors are found recom- 

 bined in about 50 per cent, of the offspring, and thus show 

 no linkage at all, presumably because they lie in different 

 pairs of chromosomes which segregate independently. 

 Further investigation showed that not only " white" and 

 " rudimentary," but all the known sex-linked factors, in- 

 stead of segregating independently, are ' * partially linked 

 to one another in greater or less degree. This then was 

 additional evidence that these factors did not lie in differ- 

 ent pairs of chromosomes, as in familiar cases, but in the 

 same pair of chromosomes, and that their separation or 

 recombination was therefore dependent upon chromo- 

 somal interchange. But furthermore, if these linked fac- 

 tors all lie in the X-chromosome (being sex-linked), then 

 it might be expected that other groups of interlinked fac- 

 tors also would be found, that lie in other chromosomes. 

 A factor in any one of these other groups would not be 

 sex-linked, but would be linked in greater or less degree 

 to every other factor of the same group, since it lay in the 

 same chromosome with it, but it would undergo 50 per 

 cent, of recombination with factors in other groups. This 

 expectation has been fulfilled. In 1911 Morgan and Lynch 

 found two pairs of factors in Drosophila (black versus gray 

 body color; vestigial vs. long wings) that were linked to 

 each other, but that were not sex-linked (10) . These were 

 designated as lying in group II or Chromosome II. Later, 

 Sturtevant found that two other pairs of factors (pink vs. 

 red eyes and ebony vs. gray body color) were also linked 

 to each other, but were neither sex-linked nor linked to the 



