THE NATURE AND EXPRESSION OF MENDELIAN FACTORS 137 
7 families should breed true for triangular capsules. 
4 families should give 15 triangular: 1 top-shaped. 
4 families should give 3 triangular:1 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 F2 
family of Bursa bursa-pastoris  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 F3; when a plant having the genotypic 
constitution indicated in the same square, is self-fertilized. (After Shull.) 
When three duplicate factors are concerned in a hybrid the ratio in 
F, is 63:1, with four factors, 255:1, andsoon. 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 Fy 
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 
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