The fundamental importance of Mendel’s law of heredity is generally
recognized among biologists. It is a working hypothesis whose utility
is fully substantiated by abundant results daily increasing in amount.
But biologists are not in agreement as to how much this law includes.
All perhaps would agree that it implies the existence in the germ-cell
of specific determiners essential for the production of particular
characteristics in the offspring. Further, no one probably will object
to the statement that it implies a dual or duplex condition of the
zygote as regards determiners and a simple or simplex condition of the
gamete. Thirdly, the fact will be admitted by all that most mendelizing
characters are wholly independent of each other in heredity, for which
reason we are forced to suppose that their determiners are distinct
within the germ-cell.
But beyond these few generalizations great diversity of opinion
exists. As regards the very nature and function of the determiners,
some consider them unvarying, and explain the observed variation of
mendelizing characters in organisms as due to a modifying action of other
determiners. At one time even a modifying action of other determiners
was denied, and the theory was advanced that the gametes extracted from
a mendelian cross are _pure_ as regards the single characters which
may have been concerned in that cross. Investigations carried out by
Castle have done something to dispel this idea. In particular it was
shown (Castle, 1905, 1906; Castle and Forbes, 1906) that in guinea-pigs,
polydactylism, long-hair, and rough coat are mendelizing characters which
are affected in the degree of their development by crosses—that is, when
these characters are “extracted” from crosses the characters are not
exactly the same as before; hence the gametes are not “pure.”
The experimental result is not denied, but in order to save the substance
of the theory its advocates now suppose that the determiners have not
changed, but in consequence of the cross certain modifiers have become
associated with them which change their appearance in the organism. The
real unchanging thing is now called the “genotype,” its appearance the
“phenotype.”
In this genotype theory we are dealing only with a new and more refined
aspect of the “theory of pure gametes.” It is not a necessary part of
mendelism, not even an original part; but it is very important for us to
know whether it is true or not. For if it is true, selection unattended
by hybridization is largely a waste of time, as De Vries and Johannsen
have maintained, and Jennings and Pearl have reiterated.
The investigation which we are about to describe was started six years
ago to test the validity of the theory of pure gametes which was then
current. Pure “genes” had not yet been invented. The investigation has
been in continuous progress ever since, and while we expect to continue
it further, it seems to us desirable that the results already obtained be
presented for criticism.
Some conception of the work entailed in the investigation may be gathered
from the statement that we have during its progress reared and studied
the color pattern of over 25,000 rats. A long and arduous investigation
of this kind has been made possible by a series of grants from the
Carnegie Institution of Washington made to the senior author, for which
he here makes grateful acknowledgment. Thanks are also due to Dean W. C.
Sabine, of Harvard University, for encouraging and supporting the work in
a variety of ways.