THE OEDER OF THE GENES 



123 



near the other end is the gene for speck. Bridges fur- 

 nishes the following data in regard to crossing over 

 between these loci. When only these distant loci are 

 used the crossover value is 48.7. When the sum of the 

 crossover values between the following seven genes is 

 taken as the value for star and speck it amounts to 104.4. 



star 



Dachs 



Black 



104.4 



11.8 



Purple 

 Vestigial 



Curved 



30.2 



48.7 



Speck 



In this case (as other experiments show) there are still 

 two units missing in the map distance given above (104.4), 

 because of one per cent, of double crossovers in the 

 region between curved and speck, that are not here 

 recorded, since there are here no loci within this distance 

 of 30.2. 



Whenever cases in which double crossing over has 

 taken place are checked up as in the foregoing cases, it is 

 found that the discrepancies in the two methods are 

 accounted for. 



It is instructive to compare the preceding case with 

 another one including several of the same genes, but in 

 addition something else (deficiency), that cuts down the 

 amount of crossing over in certain regions. The cross- 

 over value between star and speck was found to be 47.7 

 in this experiment. The sum of the crossover values of 

 the six loci involved gave 79.3 units. The difference 



