INHERITED LINKAGE VARIATIONS IN THE SPX'OND 



CHROMOSOME. 



By a. H. Sturtevant. 



INTRODUCTION. 



The data presented in this paper demonstrate the existence of two 

 genes that influence the amount of crossing-over in the second chro- 

 mosome of Drosophila inelanogaster (ampelophila) .^ These two genes 

 were both found in the same female, that came from a stock collected 

 in Nova Scotia. Each of the genes, in females heterozygous for it, 

 decreases the amount of crossing-over in the region in which it lies. One 

 of them (the other has not been tested) produces no appreciable 

 effect on crossing-over in females homozygous for it. Those results 

 are both paralleled by the effects produced on the third chromosome 

 by a gene in that chromosome. The latter case is discussed briefly. 

 An account is also given of a race in which the amount of cnwNsing- 

 over in one region of the second chromosome is increased. This last 

 case is not yet fully worked out. 



NOVA SCOTIA CHROMOSOME. 



The two loci, vestigial and speck, usually show about 37 per cent 

 of crossing-over, as appears from the summaries here presented by 

 Bridges and Morgan. In September 1913, the writer mated a wild 

 female, of a fresh stock collected by Miss E. M. Wallace at Livenxx)!, 

 Nova Scotia, to a vestigial speck stock that had been used in ni:ikiiig 

 the crosses reported by the writer (1915), and liad in those crosses 

 given the usual result. A single Fi female from this mating w:us 

 mated back to three vestigial speck males of the above stock to jiro- 

 duce culture 7 of this paper. The result of this mating was 55 wild- 

 type offspring and 44 vestigial specks — no cross-overs (see .Vjipendix). 

 Two of the wild- type daughters were mated to vestigial speck brothers, 

 to produce cultures 68 and 69. These produced 2 cross-overs among 

 136 and 2 among 120 offspring, respectively. The same tyi^e of mat- 

 ing was repeated in the next generation, in cultures 104, 105. 106, 110, 

 113, and 114. In 104 and 105 great difficulty was cxperiencetl in 

 classifying speck (the only time I have ever noticed such a difficulty 

 with this character), and the two cultures were unfortunately dis- 

 carded without any attempt being made to see wherein the difliculty 



'A preliminary note on this case has already been published (Sturtevant. 1917). It h»* nlto 

 been discussed by Morgan, Sturtevant. Muller, and Bridges (1015). MuUcr (191fi). and dwwher*. 



. 307 



