Mapping Genes on Chromosomes 181 



only way it could be plotted would be to the other side of px 

 from that on which vg lies, as: 



33.5 6.5 



vg px sp 



40.0 



Four of the six genes are now^ plotted. If gene c, when tested 

 with pr and sp in a three-point cross, showed 21.0 per cent of 

 recombinations with pr and 31.5 with sp, and if now pr and sp 

 appeared to be 52.5 map units apart, since pr is 46 units to one 

 side of px and sp is 6.5 units to the other side, gene c would be 

 placed in the following position: 



21.0 31.5 



pr vg c px sp 



sis 



As the genes used for this three-point test were rather far 

 apart, the position of c could be further tested in a three-point 

 cross involving vg and px. If b were now tested, it could be 

 placed in position. Not knowing where it might be located, we 

 might try it first with vg and sp. If the recombinations were 

 bvg = 18.5 per cent, vg sp = 40.0 per cent, and b sp = 58.5 per 

 cent, after corrections were made, b could be placed only to the 

 left of vg and 18.5 units aw^ay, so that the extended map would 

 become: 



18 .5 400 



b pr vg c px sp 



58.5 



In mapping the genes, b would be placed at the extreme left 

 end of the chromosome at locus 0; then, if the appropriate units 

 to the right were counted off, each gene would be placed in 

 position the correct number of units from its nearest genes. 

 Actually, many more than these six genes have been found on 

 the second chromosome of Drosophila melanogaster, and a num- 

 ber have been found even to the left of gene b. As new genes 

 are discovered and located, the maps must be continually re- 



