190 



CHAPTER 22 





"Triple +" 



y"^ f"^ 12345612345612 3456 <^°'' 



y+ ^4-r 

 I I 



123456 123456 



y+ f 



123456 123456 ^"''^ 



MEIOTIC 

 PRODUCTS 



FIGURE 22-3. Production of Bar chronwsomes by crossing over in Ullrahar females. 



in the cis position. Note that the trans het- 

 erozygote and cis heterozygote have the 

 same number of genes on each chromosome, 

 but, in the former case, a has b^ for its Hnear 

 neighbor (and a'' has b) whereas in the latter 

 case a has b for its linear neighbor (and a+ 

 has 6+). If the trans dihybrid has one pheno- 



typic effect and the cis dihybrid, obtained 

 from it by crossing over, has a different 

 phenotypic effect, then position effect will 

 be considered proven if the cis form can, by 

 crossing over, revert to the trans form and 

 restore the old phenotypic effect. 



This cis-trans test for position effect should 



