is a difference of only one unit involved in 

 the cross. Thus, allelomorphs ABCDEFQE 

 may determine a single characteristic and 

 ABCDEFGh an alternative characteristic. If 

 plants having characters so determined are 

 crossed together, they will behave as if these 

 were unit characters, though according to our 

 assumption one is determined by the presence 

 of eight dominant units, the other by seven. 



The best actual examples we now have of 

 the compound nature of certain apparently 

 simple external characters are seen in the 

 splendid results of Professor Bateson's studies 

 on stocks and sweet-peas. In stocks, for in- 

 stance, canescence is found to depend upon the 

 simultaneous presence of three dominant al- 

 lelomorphs wholly uncorrelated and each act- 

 ing in the normal Mendelian manner. In one 

 strain of sweet-peas two such dominant units 

 are necessary to produce any color whatever 

 and another unit determines whether that 

 color shall be blue or red. This condition 

 produces the remarkable result that the first 

 generation hybrid between two white-flowered 

 parents have blue or red flowers. 



Similar conditions were presented in two of 

 the papers given yesterday (December 28, 

 1906) on the joint program of Sections F and 

 G of the American Association for the Ad- 

 vacement of Science, viz., the appearance of a 

 'latent 7 agouti factor in certain guinea-pigs, 

 and an invisible red factor underlying black 

 in certain fowls as reported by Dr. Castle* and 

 Dr. Davenport. 4 The characters of both these 

 apparently anomalous hybrid products were 

 recognized as atavistic or reversionary. The 

 same is true of the purple-flowered hoary stocks 

 produced from glabrous white and glabrous 

 cream-colored strains. The same was true of 



Castle, W. E., 'On a Case of Reversion In- 

 duced by Cross-breeding and its Fixation.' 

 'Davenport, C. B.. 'Reversion.' 



