256 Sturtevant. 



it "works" — is certain. The phenomena of interference described and 

 discussed above are in exact agreement with what would be expected 

 on the view that crossing over is due to a twisting of the chromosomes 

 followed bj' breaking and recombination (chiasmatype of Jakssens). 



Special Cases. 



There are at least two special cases, aside from the phenomena 

 of linkage, where cytological investigation has helped to clear up difficult 

 problems in genetics by giving a simple chromosome explanation. One 

 of tlie few clean cut results in the intricate series of phenomena found 

 in the genus Oenothera is that regarding the relation between 0. La- 

 marckiana and its mutant 0. (jigas. It was found by Lutz ('07) that 

 Lamarckiana has 14 chromosomes, gigas 28. This fact, which has been 

 fully substantiated by other workers, offers a simple explanation of the 

 differences between the two forms and of the sterility of their h}'brids, 

 the cytology of which has been studied by Geeets ('11) and Lutz ('09). 

 There are other variations in chromosome number in this group of plants, 

 and these have been investigated by Lutz ('12). She writes: "While 

 it is a very common experience to find two plants with identical chromo- 

 some numbers differing conspicuously in their vegetative characters, 

 I have never observed a single in-stance of two plants having the same 

 vegetative characters in all stages of development, differing in chromo- 

 some number." 



Fedeeley ('11) found that in certain hj'brids between different 

 species of the moth Pijgaera there appeared to be no segregation, Fs being 

 uniform and bearing nearly the same characters as Fi. Cytological 

 examination (Fedeeley '13) revealed the cause: in the F, hybrids there 

 is no reduction division, each gamete receiving the diploid chromosome 

 group. Therefore there is no chance for segregation, and Fa will differ 

 from Fl only in being tetraploid instead of diploid. 



A very remarkable series of phenomena in Drosophila reported by 

 Bridges ('13) receives a simple explanation on the assumption that the 

 two sex chromosomes in certain females do not always separate in the 

 reduction divisions, so that eggs with two sex chromosomes or with 

 none may be formed. Should this explanation be verified by cytological 

 examination it will, I think, afford an almost complete proof that the 

 sex-linked genes involved are borne by the sex chromosomes. 



