Chapter 22 

 QUANTITATIVE CHARACTERS 



It was shown in the last chapter that when two duplicate, 

 cumulative genes which lack dominance are interacting to pro- 

 duce a colored grain in wheat, the ordinary dihybrid ratio takes 

 on a different appearance. If, for example, a plant with very 

 deep red grains due to four genes for red is crossed with a white- 

 .grained type, the Fi will be intermediate because it contains 

 only two genes for red, and the F2 will segregate into a ratio of 

 1:4:6:4:1, with the intermediate type most numerous and 

 the extremes resembling the two parents, and considerably less 

 frequent. The depth of color in each class depends upon the 

 presence of a certain number of positively acting genes, the sym- 

 bols of which can be designated by capital letters. Since one 

 allele is not dominant over the other, the terms "dominant" and 

 "recessive" cannot accurately be used to describe these dupli- 

 cate, cumulative, nondominant genes. According to this theory, 

 one gene adds something to the strength of the expression of 

 the character, whereas its allele neither adds nor subtracts from 

 that strength. Therefore, we can designate the former type as a 

 contributing gene and the latter as a neutral gene, and we can 

 use capital letters for the contributing genes and lower-case let- 

 ters for their neutral alleles. In Nilsson-Ehle's wheat, Ri and R2 

 would be contributing genes, and ri and r-z would be their neutral 

 alleles. The color depends entirely upon the number of con- 

 tributing genes possessed by a given plant. If one of these 

 wheat plants has two such genes, it is intermediate in color, 

 and it matters not whether its genotype is R-Jli roro, riri R2R2, 

 or RiriR2r2', no other combination of genes would produce an 

 intermediate color. 



If three pairs of duplicate, cumulative, nondominant genes are 

 interacting, the Fi will again be intermediate; but the ratio in 

 the F2 will become 1 : 6 : 15 : 20 : 15 : 6 : 1. Out of 64 plants, 

 one will be as dark as the dark parent, one will be colorless, 



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