Complete Linkage and the F2 



161 



Complete Linkage and the F2 



When linkage is complete, the F2 ratios are different from 

 the 9:3:3:1, and are also different depending on whether 

 the original cross was made in the coupling or in the repulsion 

 phase. In testing to determine whether the gene hullata, bu, 

 which causes the rosette leaves to be greatly crinkled in the 

 evening primrose was linked with the gene for old-gold flowers, 



P,: 



F, 



Coupling 



u bu 



Repulsion 



vBu 



VBu 



vbu 



VBu vbu 



3 VBu 

 Vbu 



vBu 



1 u bu 



Vbu 

 vBu 



Vbu vBu 



2VBu 

 1 Vbu 

 1 vBu 

 vbu 



Fig. 54. F2 ratios obtained between two genes that appear to show 

 complete linkage. Both the coupling and repulsion phases are indicated. 



V, G. H. Shull crossed a yellow-flowered bullata plant with an 

 old-gold flowered nonbullata. Yellow flowers, F, are dominant 

 to old-gold, V, or vetaurea, and the bullata gene is recessive to 

 nonbullata, so tlfat this cross was made in the repulsion phase, 

 V hu / V huy^ V Bu / V Bu. If there were no linkage between 

 hu and v, the r2 would segregate into 9 V Bu : 2>V hu : Zv Bu : 

 1 V bu. The actual results was in the ratio of 2 F Bu : IV bu : 

 1 V Bu : V bu. How could such a ratio be obtained? 



If the genes v and bu were linked and if they were so close 

 together on the chromosome that a break would be unlikely to 

 occur between them, what would be the result in the F2 if the 

 original cross was made with the parents in the coupling phase 

 and would it be the same if they were in the repulsion phase? 

 As shown in Fig. 54, when these completely linked genes enter 

 in the coupling phase, the Fo ratio becomes 3 T' Bu : 1 v bu ; but 



