LINKAGE, CROSSING-OVER, AND THE GERM PLASM 425 



further support. The opponents of the hypothesis are yearly becom- 

 ing fewer and fewer, and the few remaining irreconcilables are having 

 less and less to say. It should be said, then, in all faii'ness that the 

 hypotheses discussed in this chapter have been most fruitful in leading 

 to new discoveries, and in last analysis this is the only fair test of a 

 hypothesis. If it is fruitful, it is good. 



The crowning feat of the Drosophila workers is the making of the 

 chromosome maps of the species studied. While it is impossible to 

 obtain the latest version of the map, for the reason that new loci are 

 continually being added, the accompanying map (Fig. 85) gives the 

 locations of the genes that have been determined most carefully. It 

 will be noted that not only have the genes in the X-chromosome been 

 located, but also those in the other three chromosome pairs. A few 

 additional situations that have arisen out of the studies involved in 

 making the map will now be discussed, and then this somewhat diffi- 

 cult chapter will be brought to a close. 



Multiple allelomorphs. — ^In a previous connection we have dis- 

 cussed the multiple-factor hypothesis as an explanation of quantita- 

 tive heredity. Multiple factors are duplicate factors located in differ- 

 ent chromosomes. Quite definitely in contrast with that situation is 

 one in which different factors or different forms of the same factors 

 occupy the same locus of the same chromosome. For example, red 

 eye in Drosophila is a single factor. A change in the red-eye factor 

 gives white eye; another change in red gives cherry; another gives 

 eosin; and several other definite mutant colors resulting from changes 

 in red have been observed. Now each of these changed color factors 

 is an allelomorph of red and each is also an allelomorph of any of the 

 others. By this we mean that, if a cross is made between individuals 

 differing with respect to any two of these alternative colors of eye, one 

 will be dominant over the other in the Fj generation, and there will be 

 three dominants to one recessive in the F2 generation. One of the 

 assumptions about allelomorphic genes is that they occupy equivalent 

 locations in homologous chromosomes. This can be put to a crucial test. 

 No more than two members of a set of multiple allelomorphs can be 

 present in one individual because there are only two homologous 

 chromosomes and hence only two equivalent gene loci. This proves 

 to be true, for when red eye and white eye enter into a cross only these 

 two eye colors come out of it; when cherry and white go in, only cherry 

 and white come out; when red and cherry go in, only red and cherry 

 come out. Several other authors have found interesting sets of multi- 



