VAEIATION IN LINKAGE 143 



Not only external factors but internal factors, and 

 these genetic ones, may influence the amount of crossing 

 over that takes place. Sturtevant has discovered two such 

 genes in the second chromosome of a certain stock of 

 Drosophila. A female from a wild stock from Nova 

 Scotia was crossed to a male showing the characters ves- 

 tigial and speck. One of the daughters was tested and 

 gave no crossovers in 99 offspring, though the vestigial, 

 speck hybrid usually gives about 37 per cent, of crossing 

 over. AH of the descendants of this female that were 



% ^ y ^ _ I =p 



S'5, IF'^.'P- 





ST* 



)2: %-A -^ =f 



42.4 4aa ■^•-..^^ 





<£« 



>'o 9U «Lr aJ.t 



FzG. 57. — Diagram illustrating the effect on crossing over due to the presence of crossover 

 genes. (After Sturtevant.) 



known, through linkage relations, to have the Nova Scotia 

 second chromosome, gave the same result, while those of 

 her descendants that did not have the particular chromo- 

 some did not show such a change in linkage. These rela- 

 tions held regardless of whether the chromosome involved 

 had come from the father or the mother. 



A number of experiments were made with females hav- 

 ing a Nova Scotia second chromosome, while the other 

 second chromosome bore the mutant genes for black, pur- 

 ple, curved, and in other experiments other mutant genes 

 were present. In Fig. 57 (upper line) all the genes stud- 

 ied, viz., star {S), black (6), purple {pr), vestigial {vg), 



