MATERIAL AND METHODS.
In June 1906 Dr. Hansford MacCurdy completed, under the direction of the senior author, a study of the inheritance of color in rats. His studies had shown that the piebald pattern of “hooded” rats behaves as a mendelian recessive character in relation to the uniform or nearly uniform coloration of wild rats, but that the hooded pattern, when extracted from a cross with wild stock, shows a different variability, the pigmentation of the extracted recessives being increased in extent. This result was interpreted as showing the unsoundness of the current doctrine of “purity of the gametes” in mendelian crosses.
Upon the conclusion of Dr. MacCurdy’s experiments, the pedigreed stock which he had used was not entirely discarded. A certain portion of it was utilized for new experiments designed to show whether the “hooded” coat-pattern can be modified by selection unattended by cross-breeding.
Two series of selections were started in October 1907, in one of which animals were chosen as parents which had pigmentation as extensive as possible. This we may call theplusseries. In the other series animals were chosen as parents which had pigmentation as restricted as possible. This we may call theminusseries.
During the academic year 1906-7, the experiments were in immediate charge of Mr. W. G. Vinal; during 1907-8 the plus series was in charge of Mr. H. S. Rand, while the minus series was in charge of Mr. F. C. Bradford. Throughout this time the experiments were closely supervised by the senior author, who assisted in the “grading” of every litter of young. In October 1908 the junior author began his association in the experiments, which has continued up to the present time. Throughout these five years he has looked after the details of the experiments almost continuously, but both authors have in most cases taken part together in the grading of the young, and in no case has the grading been done except under the immediate supervision of one or the other of the authors. This fact is stated to show that the personal element in the grading has been kept as constant as possible. In the tabulation of results and computation of statistical constants, the authors have worked together. This statement of results is written by the senior author.
During the year 1906-7 the young rats were graded by the method used by MacCurdy and Castle (1907) that is, the back-stripe was measured and a calculation made of the percentage of the dorsal surface posterior to the hood which was pigmented. But on account of the irregular outline of the back-stripe in many individuals the method of measurement was found to be at best a rough one, as well as extremely laborious. Accordingly in the summer of 1907 a set of arbitrary gradeswas adopted, which is shown at the top ofPlate 1. Each young rat was classed in that grade which it most nearly approached in amount of pigmentation. Skins of rats graded from -3¼ to +4¾ are shown in the middle and lower rows of Plate 1. The grading was done when the rats were about three or four weeks old, at which time selected individuals were reserved as the parents for a later generation, the remainder being discarded. This method has been followed ever since its adoption and the data thus obtained are summarized in the tables, which cover the breeding operations of a little more than six years, 1907-1913.
The grouping of the young in a series of generations is only approximately accurate, for practical considerations have often led us to mate together animals which belonged to different generations of offspring. When, for example, an animal of generation 2 was mated with one of generation 4, the question would arise: To what generation do the offspring belong? In deciding this question we simply added one to the mean of the generations to which the respective parents belonged. In the foregoing case this would be (2 + 4)/2 + 1 = 4.
In case one parent belonged to generation 2 and the other to generation 3, a fractional result would be obtained, thus (2 + 3)/2 + 1 = 3½. In making up the summaries of the generations as given in the tables, offspring like the foregoing, of generation 3½, were divided equally between generations 3 and 4, alternate litters of young as recorded in the ledger being assigned to each. Offspring belonging to generations 2¾ and 3¼ were tabulated in generation 3; those belonging to generations 3¾ and 4¼ were tabulated in generation 4, etc. While, therefore, the generations as tabulated overlap, it is clear that they include groups of offspring of selected parents each the result ofone additional selection over the preceding group.
The early generations include too few individuals to be of much statistical value, but where the number of offspring rises to 500 or over, the statistical constants acquire undoubted value. The data have been given in the form of correlation tables which will repay careful study. In the tables a single entry has been made for each individual offspring in that row which corresponds with themean gradeof its two parents. Thus, if one parent were of grade 2 and the other of grade 2½, the offspring would be entered in the row 2¼ along with the offspring of parents both of grade 2¼. Offspring of parents whose mean grade fell between the rows given in the tables were divided equally between the adjacent rows, alternate litters being assigned to each. Thus, if the mean grade of the parents were 2⅟₁₆, alternate litters of offspring would be entered in row 2 and in row 2⅛.