176 . Locating Genes on Chromosomes 



AVhen the testcross phenotypes are tabulated with respect to 

 the genes bm and pr only, these results are obtained: 



Noncrossovers Crossovers 



+ + (+)— 232 + pr (y)— 84 



bmpr{v)—2S5 bm + (+)— 77 



+ + (v)— 201 + pr (+)— 40 



bmpr(+) — 194 bm + (v) — 46 



862 247 



1109 



22.27 per cent 



There are 22.27 per cent of recombinations between these 

 genes. When genes pr and v only were considered, these results 

 were obtained: 



Noncrossovers Crossovers 



{+) + +—232 (+) + v— 201 



(bm) pr V — 235 (bm) pr + — 194 



(+) pr v — 84 (+) pr + — 40 



(bm) + + — 77 (bm) + v — 46 



628 481 ,,^^ 



= 43.37 per cent 



1109 



Thus there are 43.37 per cent of crossover phenotypes when 

 this testcross is made. Now if only genes b7n and v are con- 

 sidered and gene pr is omitted, and only the Bm V, bm v, Bm v, 

 and bm V phenotypes are tabulated, the results are: 



Noncrossovers Crossovers 



+ (+) +—232 + (pr) V— 84 



bm (pr) y— 235 bm (+) +— 77 



+ (pr) +— 40 + (+) V— 201 



bm (+) V — 46 bm (pr) + — 194 



553 556 



1109 



= 50.14 per cent 



According to this, there should be 49.86 per cent of parental 

 combinations and 50.14 per cent of recombinations. If these 

 three genes are plotted on a chromosome, using one per cent of 

 crossovers as one map unit, the resulting map appears as: 



