Piebald rats and selection

by William E. Castle; John C. Phillips

The mutant female had 16 young, 6 in the lower group, mean 3.87, and 10 in the upper group, mean 5.60. (See [Table 51], lowest row.) The other females had in all 114 young almost equally divided between the two groups, 58 in the lower group, mean 3.73, and 56 in the upper group, mean 5.45. This result indicates clearly (what the sequel also confirms) that the male mutant transmitted in half his gametes the high grade of pigmentation which he himself manifested, while in the other half of his gametes he transmitted the ordinary condition of the plus race at that time. In other words his “mutant” character behaved as a dominant unit in relation to the ordinary condition of the plus race.

It is evident that the female mutant was of similar constitution. This being the case, we should expect three-fourths of the offspring of the two mutants to be in the upper group. In reality 10 of their 16 young were of this sort.

The male mutant was mated also with females of the minus series with the results indicated in [Table 52]. Again, the offspring fall into two distinct groups, a lower and an upper. The lower group should be comparable with the result obtained in F₁ when the plus and minus races are crossed with each other. (Compare [Table 50].) Such it proves to be. It includes 35 individuals of mean grade -0.49 and standard deviation 0.77. Series 2 of Table 50 is nearly contemporaneous with this experiment. The F₁ offspring in that series were of mean grade -1 and standard deviation 0.60.

The upper group of offspring ([Table 52]) result, we may suppose, from a mutant gamete (grade about 5½) united with a narrow series gamete (grade about -2). This group includes 31 individuals varying closely about grade 4½, and with a standard deviation of only 0.31. The lower average grade of this group (4.43) compared with the similar group of [Table 51], which had a mean of 5.47, shows the influence of the minus-series gamete upon the heterozygote in lowering its grade by about 1. Whether the plus-series gametes have any effect upon the grade of the heterozygotes recorded in the upper group of Table 51 is not certain, because a homozygous group of mutants has not yet been established. It may be observed, however, that one individual in the upper group of Table 51 was of grade 6 (colored all over), and it is possible that homozygous “mutants,” when obtained, will approximate that grade, as most wild rats do. Further, a comparison of Tables [51] and [53] shows that mutant heterozygotes formed by crosses with the plus series are of slightly lower mean grade than the offspring of the two mutants, among which should occur both homozygous and heterozygous mutants. It seems probable, therefore, that homozygous mutants will be found to be of somewhat higher grade than heterozygous ones.

The question early suggested itself to our minds, will these “mutants” prove to be mutants in the sense of De Vries? Will they prove to be more stable than the modifications ordinarily secured by selection in our experiments? To test this matter, we have raised two additional generations of offspring from the two mutants and have bred a second generation of offspring from each of the four groups of F₁ offspring recorded in Tables [51] and [52], derived from matings with the plus and minus races respectively.

The F₂ descendants of the two original mutants proved very similar to the F₁ descendants. (See [Table 53].) They fall as before into two groups, an upper and a lower. The former includes 30 individuals of mean grade 5.52, the latter 2 of mean grade 3.37. As the parents of this generation were taken wholly from the upper group of offspring of generation F₁, and as theoretically that group should contain 2 heterozygous individuals to one which is homozygous for the “mutant” character, it is to be expected that in F₂ more than three-fourths of the offspring will fall in the upper group. For any pair, one member of which is homozygous for the mutant character, should produce only offspring falling in the upper group; and offspring falling in the lower group should be produced only by pairs both members of which are heterozygous.

The upper group in F₂ should contain a larger proportion of homozygous mutants than in F₁, and since the parents of F₃ were chosen from this upper group of F₂ offspring, it is not surprising that the 11 F₃ offspring recorded up to this time all fall in the upper group. The mean of this upper group is remarkably constant through the three generations, and the variability of the group as measured by its standard deviation is also low, namely, 0.19. This indicates that the mutant character is a strongly dominant unit in relation to the ordinary condition of the plus series.

[Table 54] shows the character of the F₂ offspring of the original male mutant mated with females of the plus series. The lower group parents, those into which the mutant character did not presumably enter at all, produced 59 offspring recorded in the first part of Table 54. Their mean grade is 3.78 and their standard deviation 0.33. These are very close to the constants of the general plus series, which for generation 10 were 3.73 and 0.36, respectively.

The second division of Table 54 shows the character of the young produced by the F₁ parents of the upper group ([Table 51]). Such parents are supposed to have received a “mutant” gamete from their father, grade about 5.50, and a plus-series gamete from their mother, grade about 3.75. If they produce gametes of these same two sorts, their offspring should also fall into two corresponding groups; in fact they do. There are 11 offspring of mean grade 3.86 and 79 offspring of mean grade 5.50. As in the previous generation, the two groups do not approach each other in grade. The mean and standard deviation of the lower group of offspring are similar to those of the plus race. The mean of the upper group is about the same as that of their parents (upper group of offspring, [Table 51]), namely, 5.50, as compared with 5.45; their standard deviation is somewhat lower, namely, 0.15, as compared with 0.23. This result indicates that the “mutant” character and the hooded character of the plus series segregate from each other in a simple way without modifying each other appreciably. It seems possible that they contain the same modifiers (if modifiers are present) and differ merely by the main unit which we called the hooded character in the early part of this paper. Each contains a different condition of that main unit. Consequently there is no increase of variability in F₂ when these two conditions are intercrossed. This we should expect to happen, if they differed by more than a single factor.