THE NATURE AND EXPRESSION OF MEN DELI AN FACTORS 137 



7 families should breed true for triangular capsules. 



4 families should give 15 triangular:! top-shaped. 



4 families should give 3 triangular:! top-shaped. 



1 family should breed true for top-shaped capsules. 

 Shull applied this test to his cultures and obtained substantial agreement 

 with theory throughout. Fig. 62 gives a graphic summary of his experi- 

 mental results. 



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FIG. 61. Checkerboard diagram to visualize the genetic relations in a dihybrid Ft 

 family of Bursa bursa-pastoris X Heegeri, in respect to the capsule-characters. The capsules 

 figured in each square indicate by their outline their phenotype, and by their oblique 

 ruling their genotype, the gene C being represented by lines from upper right to lower left, 

 and D from upper left to lower right. Homozygotes are densely lined, heterozygotes more 

 sparsely. The ratios indicate the expectation in Ft when a plant having the genotypic 

 constitution indicated in the same square, is self-fertilized. (After Shutt.) 



When three duplicate factors are concerned in a hybrid the ratio in 

 F 2 is 63 : 1, with four factors, 255 : 1, and so on. The first case of duplicate 

 factors was that described by Nilsson-Ehle in wheat. Here the red 

 color of certain races of wheat depends on the presence of three dominant 

 Mendelian factors so that such races are to be represented by the genetic 

 formula RRSSTT and the contrasted white race by rrsstt. The Fi 

 of a cross between two such races is of a pale red color intermediate 

 between the parental red and white, and in F 2 all shades of red are 

 found from very pale to about the same depth of color as the parent 



