Duplicate genes for capsule-form in Bursa bursa-pastoris. 



101 



actual results of the experiments. The premise that dominance is com- 

 plete fixes the character of the Fi which will be uniform and indistin- 

 guishable from the dominant parent. The expectation in F 2 and F 3 in 

 the case of a dihybrid will be rendered clear by a careful study of the 

 checker-board shown in Figure 4. Since the recessive character can 

 appear only when all the duplicate genes are absent, the number of 

 such genes involved in the given cross determines the ratio of dominant 



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Fig. 4. Checker-board diagram to visualize the genetic relations in a dihybrid F 2 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 F 3 when a plant having the 

 genotypic constitution indicated in the same square, is self-fertilized. 



