10 HEREDITY IN RABBITS, RATS, AND MICE. 



tables 16 to 18 all matings between two heterozygous parents (those 

 which produce both dark and white young). In this way we get the 

 totals shown in table 28, ''white X dark." Two groups of young are 

 here shown, one dark (grade 10 or lower), the other white (grade 12 

 or higher). The numbers of individuals in these groups are 56 and 25 

 respectively (a rather poor 3 : 1 ratio); their mean grades are 5.82 

 and 14.40. (See text-figure 1, Fa, DxW.) The dark group evidently 

 includes homozygotes (mode on 5) and heterozygotes (mode on 8) 

 which overlap in the intervening region. Accordingly all facts thus 

 far noted indicate that white and dark are allelomorphic forms of 

 Dutch marking. 



One other test of this hypothesis is possible. Fi may be back- 

 crossed with the dark race. The result of such a test is shown in 

 table 29, "Fi (white X dark) X dark," and text-figure 1, Fi X D. 

 The expectation here is the formation of two groups of equal size, 

 homozygous and heterozygous dark, with modes on 3 and 7 respec- 

 tively. In reality the intervening grades are the modal ones. This 

 does not disprove segregation. The flat-topped variation curve ob- 

 served is exactly what we might expect from the combination of two 

 simple variation curves which overlap. Compare F2, Sx W, text-figure2. 



The several facts developed as regards the relation of "dark" to 

 "white" Dutch pattern are presented graphically in text-figure 1. 

 The variability of the uncrossed races is shown at the top; the races 

 are distinct, monomodal, and do not overlap in range. Immediately 

 below is shown the character of Fi. It also is monomodal and it is 

 intermediate. Next lower is shown the variability of F2. The extracted 

 white race is here seen to have a lower mode than the uncrossed white 

 race and the extracted dark race has a higher chief mode than the 

 uncrossed dark race. In other words, the extracted races reappear 

 in Fo in character mutually modified, although their extreme range is 

 the same. The mode of the heterozygotes lies between the modes of 

 the homozygotes, as in Fi just above. 



The results of back-crosses with each of the parental races are 

 showTi in the lower part of text-figure 1. In the back-cross with the 

 white race (FiXW), the 1 : 1 segregation is unmistakable; in the 

 back-cross with the dark race (FiXD), segregation is obscured by the 

 closeness to each other of the modes for heterozygous and homozygous 

 dark Dutch, which results in producing a composite flat-topped curve. 

 Everything indicates that dark and white are simple allelomorphs, but 

 are quantitatively fluctuating and mutually modify each other in 

 crosses. If they were not allelomorphs, individuals which contained 

 neither form of Dutch should appear in F2. None such is produced. 



We may next consider whether white Dutch is a simple allelomorph 

 of self or is composite. The pertinent facts are recorded in tables 27 

 to 29 and are shown graphically in text-figure 2. The same white 



