RULES AND RATIOS OF INHERITANCE 223 



the ratio 15 to i. They occur mainly in plants. In certain 

 cases there are indications of a ratio of 63 dominants to i 

 recessive in F2. This is evidence that the dominant condition 

 is produced if any one of three dominant genes, A, B, or C, 

 is present, while the recessive condition occurs only in indi- 

 viduals of the constitution aabbcc. 



12. "Blending Inheritance." Intermediate Characters. 

 "Multiple Factors." 



There are many cases in which a cross between two dif- 

 fering stocks results in progeny (Fi) that are intermediate 

 between the two parent stocks, and this intermediate con- 

 dition may continue, in large measure, in later generations. 

 Such intermediate or blending inheritance is found mainly 

 in characters that grade gradually from one extreme to 

 another: dimensions, degrees of color, and numerical char- 

 acters, such as numbers of leaves in certain plants. A cross 

 between large and small individuals commonly results in 

 progeny that are intermediate. Crosses between whites and 

 negroes give offspring that are intermediate in color. 



It was at first supposed that such "blending" inheritance 

 is of a fundamentally different type from Mendelian inherit- 

 ance, that is, from inheritance depending on differences 

 among the genes. This was an almost necessary conclusion 

 when it was believed that each Mendelian character depends 

 exclusively on a single factor or gene. But since it has been 

 discovered that every character depends on many genes, and 

 that parents may differ in one or many genes affecting a par- 

 ticular character, so-called blending inheritance has lost its 

 unique character, and has been recognized as resulting, like 

 other inheritance, from differences between the genes of the 

 two parents. Many intermediate conditions have been dis- 

 covered between ordirtary single gene inheritance and the 

 phenomena of blending inheritance. These, with the evidence 

 for the dependence of blending inheritance on gene differ- 

 ences, are set forth in the following. 



