Double Crossing Over 



175 



given genes are. It is well illustrated by data of Rhoades (sum- 

 marized by Emerson, Beadle, and Fraser) for some genes in the 

 fifth chromosome of maize. Three genes, bm (brown midrib), 

 pr (red aleurone) and v (virescent or light yellow seedlings), 

 were involved. The cross Bm PrV / Bni PrV x bmpr v / bm 



c C- 



i 



c 



a- 





c- 



Fig. 55. Double crossing over. If two genes are far apart on a chromo- 

 some, as a and c, and only these genes are under observation, all the result- 

 ing chromatids may be of the parental type and the double crossover 

 types may remain unnoticed. If. however, a gene located between them, 

 as b, is also present, the double crossing over may be more readily detected. 

 For discussion, see text. 



pr V gave an Fi plant with green midrib, purple aleurone, and 

 green seedlings, and having the genetic constitution Bm Pr 



V I bm pr V or 



+ + + 



It was backcrossed to a bm pr v / bm 



bm pr v' 

 prv plant, and these testcross data were obtained: 



+ + + — 232 plants 

 bmpr V — 235 plants 



+ pr V — 84 plants 

 bm + + — 77 plants 



+ + V — 201 plants 

 bm pr + — 194 plants 



+ pr + — 40 plants 

 bjn + V — 46 plants 



Parental types = 467 or 42.11 per cent 



Single crossover between 6m and pr = 161 or 14.52 

 per cent 



Single crossover between pr and v = 395 or 35.62 

 per cent 



Double crossovers = 86 or 7.75 per cent 



