282 



THE SECOND-CHROMOSOME GROUP 



The females back-crossed to dachs black males produced the results 

 shown in table 129 (cultures 2679, 2680, 2681). 



The crossing-over between star and dachs was found to be normal, 

 but the crossing-over between dachs and black was the lowest ever 

 encountered (outside recognized linkage variations) , being only 11.1 

 per cent, while the mean calculated from 6,725 other flies was 17.8 per 



S' di 



Table 128. — Cultures of dachs-lethal stock, 



d' X b 9 



cent. This same experiment was repeated in 1917 (cultures 7083, 7085, 

 table 129) and the same result was obtained, since the crossing-over 

 between dachs and black was only 10.2 per cent. The lowest regular value 

 found for dachs black was 16.7 (found by Muller in his progeny tests.) 

 It would seem that these values, which represent a decrease of 39 

 per cent from the standard values, are sufficiently aberrant to prove 

 that the case is not to be explained by a simple lethal, linked to dachs. 



Table 129.— Pi, star dachs-lethal 9 ( j]X black d^; B.C., Fi star 9 



I — r I X dachs black (t) cf . 



There is a possible escape from the conclusion that the case is one of 

 dachs-deficiency, through the additional assumption of a ''cross-over 

 gene" which will give this specific disturbance of crossing-over. 



Sturtevant has found two, and Bridges one such mutation in the 

 second chromosome ; but none of these gives a result very closely com- 



