26 



HEREDITY IN RABBITS, RATS, AND MICE. 



"locus," in the terminology of the chromosome theory. But the physi- 

 ological and genetic behavior of English are so different that it would 

 seem improbable that they are variants of the same gene. 



Nevertheless, when English is crossed with Dutch, the two appear to 

 be either allelomorphs or closely linked, as the following results show : 

 Heterozygous English rabbits of grade 5 were crossed with "white" 

 Dutch of grades 15 to 17. Two matings were also made of the 

 homozygous English buck, 1173 (table 33, and plate 3, fig. 6), whose 

 English character was of equivalent potency with that of heterozygotes 

 of grade 5. These matings produced 26 English and 9 non-English 

 (Dutch) young, which were graded (with no great exactness) as follows : 



There was probably no real discontinuity 

 in the grouping of the English young, but 

 owing to the rough manner of grading them 

 the numbers heap up on the even grades, 

 fractional grades being neglected. The Dutch 

 young are similar in grade to the young pro- 

 duced by crossing "white" Dutch with self. 

 (Compare table 27.) But the English young 

 are much whiter than heterozygotes between 

 English and self. The latter are 4.75 to 5.00 

 in grade when produced by the same English sires. (Compare table 

 45.) But the English-Dutch heterozygotes in no case are of higher 

 grade than grade 3 and in the great majority of cases are no darker 

 than grade 2. In other words, they are of about the same grade as 

 English homozygotes. This means that a white Dutch gamete has 

 about the same whitening effect on English as another gamete of Eng- 

 lish would have. The pattern of the English-Dutch heterozygote is 

 indistinguishable either quantitatively or qualitatively from that of a 

 homozygous English animal. The Dutch is not at all in evidence except 

 as a whitening influence on the dominant English. Even in a single 

 dose it completely counteracts the darkening influence introduced into 

 the English race in the process of the cross with self and the subse- 

 quent 5 generations of selection. 



Since it has been shown that both white Dutch and English segre- 

 gate from self in monohybrid fashion, much interest attaches to the 

 inquiry whether they segregate from each other in the gametes of 

 the Fi individuals. Two methods have been employed to test this 

 matter, one being to mate FI individuals inter se, the other to back- 

 cross them with white Dutch. If English and Dutch are allelomorphs 

 (invariably pass into different gametes) nothing but English or Dutch 

 young should be produced by either mating. If, however, English 

 and Dutch are not allelomorphs, then a certain number of gametes 

 should be formed by F a individuals which are neither English nor 

 Dutch, but which are self, and a like number should be formed which 



