12 SIZE INHERITANCE IN RABBITS. 



brid ratio. The second generation consisted of 1,410 brown and 941 

 white ( = 15.0:1). Most red-grained wheats when crossed with white 

 gave monohybrid ratios. Three strains, however, gave dihybrid ratios, 

 while two were found that gave trihybrid ratios. 



From these facts the general conclusion was drawn that quantita- 

 tive differences in color can be brought about by the interaction of 

 two or more independent factors, qualitatively similar, which segregate. 

 These cases, although involving fewer character differences than were 

 assumed in the hypothetical case of peas, show a very essential agree- 

 ment with it. The number of different heritable grades of color in 

 oats and wheat are much fewer than the supposed heritable grades of 

 yellow in peas. 



East (1910 and 1911) first presented evidence of the existence of two 

 independent factors for the formation of yellow endosperm in maize. 

 Although most lines with yellow endosperm gave monohybrid ratios 

 when crossed with white, several gave very clear dihybrid ratios. The 

 third and fourth generations showed close agreements with expect- 

 ancies. Among pericarp colors East and Hayes (1911) have found 

 five different reds. Two of these look fairly similar, but when crossed 

 give whites in the second generation, in the proportions of 131 red to 

 7 white and 52 red to 2 whites. Tammes (1911) describes crosses 

 between strains of Linum with different flower colors. A dark-blue 

 and a light-blue strain when crossed with white gave simple mono- 

 hybrid ratios in F 2 . Fi was intermediate. In the case of the light blue, 

 the intermediate blue and parental type could not be easily separated, 

 but the intermediates were in greater numbers. Crosses between two 

 sorts of blue gave a second generation with a wide range of variability, 

 the intermediate shades having the greatest frequencies. It was 

 assumed that the blues used differed from each other in three units. 

 It is possible that the monohybrid ratios — where (by comparison with 

 Nilsson-Ehle's work) one would expect one dihybrid, if not a trihybrid 

 ratio — do not depend upon the presence or absence of blue but upon the 

 presence and absence of a color factor, as is the case of whites in other 

 flowers and in animals. 



The behavior of the various degrees of pigmentation of human skin, 

 from black negroid to fair types, affords a suggestive comparison with 

 the various shades of colors discussed in the preceding cases. Daven- 

 port and Davenport (1910) and Davenport (1911) and (1913) have pre- 

 sented evidence that supports the conclusion that the color of skin is 

 due to various factors to whose cumulative effect the successively darker 

 grades are due. In crosses between races with different grades of color 

 these units segregate in the germ cells, but on account of their number 

 the segregation is often seemingly imperfect and the parental characters 

 are only more or less perfectly restored. Mudge (1909) gives a case 

 of such segregation in which a "white" appeared in a quadroon family. 



