Gene Arrangement; Crossover Maps 



135 



gene is changed relative to the other two 

 genes), and one noncrossover. The 4- 

 strand double chiasmata yields four single 

 crossover strands. Note that each type of 

 double chiasmata produces some strands 

 with a new genetic combination, that is, 

 crossover strands; each of the three differ- 

 ent types also produces a characteristic pat- 



tern of noncrossover and crossover types. 

 Moreover, the genetic products obtained 

 from each type of double chiasmata differ 

 from those obtained from a single chiasma 

 (which produces two noncrossovers and two 

 "singles"). 



In view of the preceding discussion, it 

 should be possible to learn, from the geno- 



CHIASMATA 



MEIOTIC PRODUCTS CROSSOVERS 



2-STRAND 



2 Doubles 



£ 

 J 



a / x _ b _\/_ _ c_ _ 



»\ 



2 b _/ \ __»■_. 



3-STRAND 



c 1 Double 

 c 2 Singles 

 + 1 Noncrossover 



Ia_ _b_ 



3-STRAND 



FIGURE 10-5. 



Double nonsister 

 chiasmata types 

 and their genetic 

 consequences. 





+ + 



y 



b !\ 



4 Singles 



4-STRAND 



