44 A. H. STURTEVANT 



only if 'crossing over' is possible; which means, on the assumption 

 that both of these factors are in the sex-chromosomes, that an 

 interchange of materials between homologous chromosomes occurs 

 (in the female only, since the male has only one sex-chromosome). 

 A point not noticed at this time came out later in connection 

 with other sex-linked factors in Drosophila (Morgan '11 d). It 

 became evident that some of the sex-linked factors are associated, 

 i.e., that crossing over does not occur freely between some fac- 

 tors, as shown by the fact that the combinations present in the 

 Fi flies are much more frequent in F2 than are new combinations 

 of the same characters. This means, on the chromosome view, 

 that the chromosomes, or at least certain segments of them, are 

 more likely to remain intact during reduction than they are to 

 interchange materials.^ On the basis of these facts Morgan 

 ■('11 c, '11 d) has made a suggestion as to the physical basis of 

 coupling. He uses Janssens' ('09) chiasmatype hypothesis as a 

 mechanism. As he expresses it (Morgan '11 c): 



If the materials that represent these factors are contained in the 

 chromosomes, and if those that "couple" be near together in a linear 

 series, then when the parental pairs (in the heterozygote) conjugate 

 like regions will stand opposed. There is good evidence to support 

 the view that during the strepsinema stage homologous chromosomes 

 twist around each other, but when the chromosomes separate (split) 

 the split is in a single plane, as maintained by Janssens. In consequence, 

 the original materials will, for short distances, be more likely to fall 

 on the same side of the split, while remoter regions will be as likely to 

 fall on the same side as the last, as on the opposite side. In consequence, 

 we find coupling in certain characters, and little or no evidence at all 

 of coupling in other characters, the difference depending on the linear 

 distance apart of the chromosomal materials that represent the factors. 

 Such an explanation will account for all the many phenomena that I 

 have observed and will explain equally, I think, the other cases so far 

 described. The results are a simple mechanical result of the location 

 of the materials in the chromosomes, and of the method of union of 

 homologous chromosomes, and the proportions that result are not so 

 much the expression of a numerical system as of the relative location 

 of the factors in the chromosomes. 



1 It is interesting to read, in this connection, Lock's ('06, p. 248-253) discussion 

 of the matter. 



