THE PHYSICAL BASIS OF INHERITANCE 301 



apart of their loci. The loci for black and vestigial, for instance, are 17 

 crossover units apart, and since the one locus will contain either the gene 

 for black or the gene for gray and the other will contain either the gene 

 for long or the gene for vestigial, the crossover percentages between the 

 genes for body color and for wing length will always be 17 per cent, 

 whatever the original combination of the body color and wing length. 



