Crossing Over 157 



Crossing Over and the F2. 



So far, in discussing linked genes, only the results of back- 

 crossing the Fi to the recessive have been discussed. This is 

 usually the simplest method of determining the percentage of 

 crossing over between two genes, because the testcross ratio is 

 exactly the same as the ratio of crossover to noncrossover types 

 among the gametes. It is this gametic ratio that indicates the 

 percentage of crossing o^^er and therefore the amount of chroma- 

 tid breakage that occurred at meiosis in the Fi. Sometimes, how- 

 ever, there are practical difficulties in making backcrosses or test- 

 crosses, and it is simpler to raise an F2 by selfing the Fi plants. 



What would be the genotypic and phenotypic ratios in the F2 

 for the genes for ragged leaf and anthocyanin-1 in maize? 



Obviously the phenotypic ratio could not be 9 : 3 : 3 : 1, for 

 this ratio would be obtained only if the ratio of the gametes were 

 1 Rg A : 1 Rg a : 1 rg A : 1 rg a. The actual gametic ratio, when 

 the crossover percentage is 42, would be 0.29 Rg A : 0.21 Rg a : 

 0.21 rg A : 0.29 rg a if the original cross was Rg A / Rg A X 

 rg a/rga, and 0.21 Rg A : 0.29 Rg a : 0.29 rg A : 0.21 rg a if the 

 original cross was in the repulsion phase. The F2 could be deter- 

 mined from a checkerboard in which these gametes were repre- 

 sented in their correct ratio. For simplicity of the arithmetic let 

 us assume that the crossover percentage is 40 instead of 42. The 

 F2 from crosses in both the coupling and the repulsion phases is 

 tabulated in Fig. 52, and it is seen to be different in the two 

 phases. In each phase the ratio is different from the 9:3:3:1 

 ratio, as expressed in terms of percentage; and in both the cou- 

 pling and repulsion phases, the difference is in favor of the pa- 

 rental types. In the coupling phase, the percentage of Rg A and 

 7'g a types is respectively 59 and 9 instead of 56.25 and 6.25, 

 whereas in the repulsion phase the percentage of Rg a and rg A is 

 21 for each type instead of 18.75. 



The specific F2 ratios for coupling and repulsion that were 

 given in Fig. 52 mil be obtained only if there is 40 per cent cross- 

 ing over between the two genes in question. If the percentage is 

 greater or less than 40 the F2 ratios will be different from those 

 listed, for the ratios mil actually differ according to the percent- 

 age of crossing over. We often find it advantageous, therefore, 

 to state the F2 ratio in general terms. 



If we let p represent the percentage of crossing over expressed 



