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 the _plus_ series. In the other series animals
were chosen as parents which had pigmentation as restricted as possible.
This we may call the _minus_ series.
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 grades was adopted, which is shown
at the top of Plate 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 of _one 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 the _mean grade_ of 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.