8 HEREDITY IN RABBITS, RATS, AND MICE. 



dark and of the tan type which are given the same grade are not 

 exactly aUke in pattern. The grade is assigned to express roughly 

 the total amount of white-spotting on the animal. A rabbit of grade 3 

 in the tan series will usually have a whiter head with a broader white 

 spot on the nose but with a narrower collar than one of the same 

 grade in the dark series. With this explanation it may be stated that 

 tan Dutch rabbits bred with each other have produced 40 Dutch 

 young with the grade distribution shown in table 21. 



The variation is close about grades 3 and 4, the race being very 

 uniform in character for a white-spotted race. In origin it is derived 

 from a single gamete which introduced the character as a recessive 

 into the original yellow ancestor. 



Having now secured 3 distinct strains of Dutch rabbits, it was our 

 next task to determine what were their genetic relationships to each 

 other, whether they were allelomorphs or due to wholly independent 

 factors; whether due to single or to multiple genetic factors, and 

 whether these factors were constant or variable. Before these ques- 

 tions can be intelligently discussed the variability of each type by 

 itself must first be known. That of the tan race has just been referred 

 to. It is shown in table 21. It will be observed that the mean grade 

 of the young has a tendency to rise with the grade of the parents. 



The variation of the uncrossed "dark" race is shown in table 20. 

 The same homozygous buck (6701, plate 2, fig. 20) was mated with ten 

 different homozygous dark does ranging in grade from 2 to 5. They 

 produced 172 young ranging in grade from 1 to 7, mean 3.30. The 

 variation is shown graphically in text-figure 1, d. The higher-grade 

 mothers, it will be observed, produce higher-grade young, although 

 the differences are not striking. 



The variation of the uncrossed "white" race is shown in table 19 

 and text-figure 1, w. Two bucks of grade 17 were mated with 8 does 

 of grade 15, 16, or 17; they produced 59 young with the same range 

 of variation as the mothers and of the mean grade 16.25. Again we 

 observe a tendency for the higher-grade mothers to produce the 

 higher-grade young. 



The same homozygous white buck (6175) which was used in matings 

 recorded in table 19 was mated also with 5 homozj^gous does of the dark 

 race, with the results shown in table 22 and text-figure 1, Fi, DxW. 

 These matings produced 28 young of mean grade 7.28. All the young, 

 from their parentage, should be heterozygotes between white and dark 

 Dutch, like the original animals from which these two races were 

 isolated. In reality they agree closely with the foundation stock in 

 grade. 



Table 23 shows the results of matings of homozygous white bucks 

 (one of which was the same individual, 6175, as sired the young of 

 tables 19 and 22) with dark does which were heterozygous for white 



