SHORTER ARTICLES AND DISCUSSION 



MODIFICATIONS OF THE 9:3:3:1 RATIO 



A. Chemical Experiments Paralleling the Several Possible 

 Modifications of the Mendelian F 2 Di-hybrid Pheno- 

 typic Non-blending Ratio 9:3:3:1 



The foundation F 2 di-hybrid ratio 9:3:3:1 consists of 9 in- 

 dividuals having somatic traits both ''A" and "B," 3 individ- 

 uals having ''A" only, 3 having "B" only, and 1 having neither 

 •'A" nor "B." Or, if each allelomorphic pair consists, not in a 

 gene and its absence, but in genie entities contrasted in quality 

 or quantity and showing clean-cut dominance and recessiveness, 

 in 9 "A" and "B" both dominant; 3 "A" dominant, "b" re- 

 cessive; 3 "a" recessive, "B" dominant; 1 "a" and "b" both 

 recessive. This di-hybrid ratio was one of the early discoveries 

 of Mendel 1 himself, but after the revival of genetical studies in 

 1900 experimental breeding had not continued long before modi- 

 fications of this ratio became apparent. Thus Bateson 2 men- 

 tions a number of cases in which the 9:3:4 F 2 ratio is found. 

 It is apparent in such cases that two unit trait-pairs are in- 

 volved, that the dominant phase of one of them standing without 

 that of the other in 3 individual F 2 somas is not to all patent as- 

 pects different from the 1 individual possessing the dominant phase 

 of neither of the two trait-pairs involved. In the gametes of in- 

 dividuals of families that produce the 9:3:4 ratio the segrega- 

 tion and recombination of genes are, however, just as clean-cut 

 and follow the same rule as in pedigrees giving the unmodified 

 foundation ratio 9:3:3:1; only the somatic working out of the 

 genes is different in the two ratios. 



Barring blending, linkage, crossing-over, non-disjunction, sex- 

 limited inheritance and other special phenomena, which limita- 

 tions preserve intact the numerical entities 9, 3, 3, and 1, the 



i Mendel, G-., "Experiments in Plant Hybridization" (reprinted in Eng- 

 lish in Bateson 'a "Mendel's Principles of Heredity," pp. 334-379), p. 

 351, 1866. 



2 Bateson, W., "Mendel's Principles of Heredity," p. 80, 1913. 

 353 



