The Behavior of the Chromosomes as Studied throuffh Linkage. 



245 



Sp (speck). The spsp flies have a small dark speck at the base 

 of each wing. The mutant was described by Morgan ('10a). At that 

 time its inheritance was reported to be irregular, but I have found the 

 character to behave as a simple Mendelian recessive. It is sometimes 

 rather difficult to separate from the normal in very small light-colored 

 flies, but even this difficulty T have found insuperal)Ie only once. It 

 now seems likely that the iri'egularities found by MoRCiAN may have 

 been due to failure to use virgin females, since at that time it was not 

 realized that females may be fertilized before they are 24 hours old. 



MoKGAX ('12 d) showed tiiat in his experiments there was no 

 crossing over in the male between B and Vg, and SturtkVjVNT and 

 Bridges showed the same in their results from B and Cv. I have 

 made only one other direct test of this point. Forty-six flies from a back- 

 cross test of a male (table XIX) gave no crossing over between Cv 

 and Sp, though in the female these loci gave 26*0''/o of cross-overs. As 

 will appear below, this cross involves the other end of the chromosome 

 from that studied before. This lack of crossing over in the male pre- 

 vents the appearance of double recessives in F2 when two single reces- 

 sives are used in Pi. This result is shown in the F2's from black 

 X balloon and from curved X speck, recorded in table XIX. The numbers 

 are small, but are important in that they involve longer distances than 

 the cases previously published. We may conclude that crossing over 

 in the male if it occurs at all is at least very much less than that in 

 the female in all parts of the chromosome here treated. 



Table VI. 



') There is a misprint in Mouuan's paper ('12d), so that the data as published 

 give 25'ö°/o cross-overs. 2r97oi as given in the text, is the percentage actually obtained. 



