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[Reprinted from SCIENCE, N. S., Vol. XLIL, No. 1079, Pages 800-80S, September 3, 1915]

ARE EECESSIVE CHARACTERS DUE TO LOSS?

SINCE the presence-absence theory came into
vogue it has become quite customary to re-
gard recessive characters as due to the absence
of something in the germ plasm on which the
corresponding dominant character depends.
The nomenclature of the presence-absence
theory has been adopted by most writers on
Mendelian inheritance, and it has afforded a
useful and convenient method of expressing
gametic formulas, although, as Morgan has
shown, there are cases in which it leads to in-
consistent results. While it is often recog-
nized that this nomenclature is a purely sym-
bolic scheme of indicating how certain char-
acters behave in inheritance, the habitual em-
ployment of the system in the search for form-
ulas which will designate by a series of large
and small letters the gametic constitution of
the organisms one is dealing with, has a strong
tendency to influence one's views in regard to
several important problems of heredity and
evolution. I can not but think that the opin-
ions of many students of genetics have been
unduly influenced by their formulas. Form-
ulas are excellent servants but bad masters.
Almost involuntarily a certain interpretation
is attached to their symbolism which is apt to
have the practical effect of actual belief if it
does not succeed in producing it.

Since the establishment of Mendel's law and
its successful employment in elucidating many
previously enigmatical phenomena of inherit-
ance, heritable variations have commonly
come to be considered as due to the addition or
subtraction of discrete units of germ plasm,
the bearers of unit characters. Professor
Bateson in his "Problems of Genetics" says
in regard to substantive variations that

we are beginning to know iri what such variations
consist. These changes must occur either by the
addition or loss of factors.

And further on he makes the following sig-
nificant statement :

Eecognition of the distinction between dominant
and recessive characters has, it must be conceded,
created a very serious obstacle in the way of any
rational and concrete theory of evolution. While
variations of all kinds could be regarded as mani-
festations of some mysterious instability of organ-
isms this difficulty did not occur to the minds of
evolutionists. To most of those who have taken
part in genetic analysis it has become a permanent
and continual obsession. With regard to the origin
of recessive variations, there is, as we have seen,
no special difficulty. They are negative and are
due to absences, but as soon as it is understood that
dominants are caused by an addition we are com-
pletely at a loss to account for their origin, for we
can not surmise any source from which they have
been derived.

In his more recent address before the Brit-
ish Association, Bateson not only interprets
all recessive characters as due to loss, but sug-
gests that dominant characters may have
arisen by the removal of inhibiting factors,
thereby causing a " release " of the characters
which previously lay latent in the germ plasm,
and producing the appearance (but only the
appearance) of new variations. He says:

In spite of seeming perversity we have to admit
that there is no evolutionary change which in the
present state of our knowledge we can positively
declare to be not due to loss.

If we explain not only the actual disappear-
ance of characters as caused by germinal loss,
but the appearance of new characters as due to
the loss of inhibitors which prevented these
characters from manifesting themselves, it is
theoretically possible to consider the whole
process of progressive evolution as accom-
plished by the sloughing off of inhibiting fac-
tors. Such a doctrine which naturally re-
minds one of the extravagancies of the theory

342603

H

SCIENCE

of emboitement might have proved quite ac-
ceptable to Leibnitz, Haller, or Bonnet, but,
unless I misunderstand him, Professor Bate-
son has presented this view more as an illus-
tration of the bankruptcy of present evolu-
tionary theory than as a matter of serious
conviction of his own. I will not discuss this
interesting speculation further than to observe
that any interpretation of variation which
logically leads to such a standpoint naturally
incurs a very justifiable suspicion of unsound-
ness. It may be that in the case of any par-
ticular variation we are unable to positively
declare that it is not due to loss, but on the
other hand we are unable to positively declare
that most variations are due to loss. I think
I am not going too far in stating that a ger-
minal variation due to loss has not been
proved to occur in any single case. If it is
legitimate to explain the appearance of new
characters as due to the removal of inhibitors,
we may also explain the apparent loss of a
character as due to the advent of inhibitors.
It is surely justifiable to assume that inhibi-
tors can come into an organism somehow if we
t are permitted to make such frequent use of
their disappearance in accounting for the'
origin of new variations. The .plain fact is
that we know practically nothing of the
i changes in the germ plasm which we postu-
« \ late as the causes of variability. It is easy to
assume the existence of an inhibitor to bring
any particular variation into line with one's
general theory, but such explanations are
purely formal and therefore of little scientific
value.

While few would be inclined to follow Bate-
son in his rather paradoxical interpretation of
dominance, the doctrine that recessiveness is
due to loss is coming to be quite prevalent
among workers in genetics. One of the chief
reasons for regarding so much of the varia-
tion that has arisen among domestic animals
as caused by the loss of factors is the fact that
the crossing of different varieties often pro-
duces a reversion toward the ancestral type.
If we regard the ancestor of our races of do-
mestic mice for instance as possessing a full
complement of factors, and assume that the
different varieties have arisen by the dropping

out of one or more factors in this variety, and
one or more other factors in that variety, then
when these varieties are crossed the hybrid
may possess all the factors of the original an-
cestor and hence show a reversion to type.
On the basis of this assumption one can make
out gametic formulas for the different vari-
eties of a species, test them by breeding ex-
periments, and thus verify their correctness.
Gametic fomulas obtained in this way doubt-
less symbolize a truth in regard to the germi-
nal constitution of the organisms in question.
The value of such formulas is no longer a
matter of doubt, and is quite independent of
the various interpretations that can be made
concerning the nature of the symbolism, just
as chemical formula? are of value quite irre-
spective of the various theories of the consti-
tution of atoms.

Consider the origin of a black mouse ac-
cording to the presence-absence hypothesis.
We may explain the origin of a black mouse
by saying that it is caused by the absence of
the agouti or ticking factor that breaks up
the color of the hair into bars. Gray is there-
fore black plus an agouti factor. But does it
' J 'follow that because we can interpret the facts
in this way, and interpret them consistently so
far as breeding experiments are concerned,
the change that has taken place in the germ
plasm that produced** a black mouse was
really a loss? Such a change is frequently
assumed to be the result of an actual loss of
a little discrete unit of some sort in the germ
plasm. De Vries has interpreted recessiveness
as due to the latency or loss of potency of pan-
gens, but we may also assume that the germi-
nal basis of the character in question has
undergone a change of such a character that
without becoming inactive it ceases to func-
tion in its usual way. The agouti factor
(commonly designated by G) may be regarded
as dependent on a part of the germ plasm, a
section of a chromosome possibly, which when
present causes the barring of color in the hair.
When a black mouse arises we may suppose
that something takes place in G. It is not
necessarily a change in the direction of either
chemical or organic simplicity any more than
it is necessarily a loss of substance. The fact

SCIENCE

that the modified condition is recessive to G
proves nothing whatsoever in regard to the
nature of the transformation that has occurred
in the germ plasm. Assuming that G is not
actually lost, but modified into another kind
of substance g, the recessiveness of g may be
due to the fact that its activity is manifested
in a different way, relative slowness of its
metabolism, or to various other conceivable
causes.

There is, I believe, no good reason for con-
sidering the recessiveness of a character as
due to the relative simplicity of its germinal
basis. Many variations of a minus character
are recessive, but there are numerous excep-
tions to this rule, as is illustrated by the domi-
nance of the hornless condition in cattle, the
short tail of Manx cats, and the lack of beards
in certain kinds of wheat. Suppose we have
two allelomorphic units (assuming for the
present that there are such things as germinal
units) A and A', one of which tends to pro-
duce a relatively simple development of a part
and the other a relatively complex development
of a particular part of the body. The one A
calls forth, say, simple horns, the other
branched horns. A and A' presumably differ
chemically, and the development of the part
in question depends not upon A or A' alone,
but upon how these agencies affect other parts
of the body during development. Will the
simpler substance or organic unit call forth
the simpler structure in the adult body? In-
asmuch as the development of any organ in-
volves activities in which a larger number of
elements are concerned it seems not at all im-
probable that the simpler substance or unit
might conspire to produce the more complex
organ. Now suppose that the forked horn
proved to be dominant over the simple horn.
What conclusions would we be entitled to draw
from this fact concerning the germinal basis
of these characters? Obviously none.

Whether we interpret a variation as a gain
or a loss is in most cases a purely arbitrary
matter. In sugar corn there is a loss of starch
but there is a gain of sugar. Does sugar corn
therefore represent a plus or a minus raria-
tion? Consider the familiar cases of rose
comb and pea comb in poultry. Both of these

variations are dominant over the primitive
condition of single comb. Yet both breeds
carry the basis for the production of single
comb in their germ plasm. It is commonly
assumed that both conditions represent single
comb plus something. We may suppose that
in a certain chromosome a change has taken
place which results in the development of rose
comb. This change, for all that we know,
may be due to the loss or impairment of a
portion of germ plasm, or it may be due to a
change not properly describable as either a gain
or loss. We may regard rose and pea comb as
more or less pathological deviations based on
germinal defect, as true progressive varia-
tions, or simply as normal variations neither
progressive nor retrogressive. So far as com-
plexity of structure is concerned it may be a
matter of dispute whether rose comb, pea
comb, or single comb represents the higher
grade of development.

But, it may be asked, are not color varieties
commonly due to loss, and is not this obviously
the case with albinism? In many varieties
there has certainly been a loss of pigment, but
has there been a dropping out of factors? It
by no means follows. The factors represented
by small letters in our color formulas are by
no means missing entities. They are changed
so that they occasion a diminished production
of certain pigments, but in other respects they
may be as potent as before. The albino does
not produce pigment, but there may be other
substances in the place of pigment that would
distinguish the albino as a positive variation
when judged by other standards. The animals
whose gametic formulas contain a number of
small letters are not necessarily more imper-
fect or perhaps I should say incomplete than
their congeners which carry a large number
of dominant characters.

Of course there may be varieties due to
losses of germinal material. Considering the
complex mechanism of mitosis, and the op-
portunities afforded for the loss of chromatin
during this process, such variations are not
improbable a priori. But there is not the
slightest warrant in the fact of recessiveness
per se for the doctrine that all recessive varia-
tions are produced by this method. The origin

SCIENCE

of so-called unit characters may depend, for
the most part, not upon germinal loss or gain,
but simply on transformation. Viewed in this
simple and natural way the appearance of a
new dominant character is not an event to be
marvelled at. Dominant and recessive charac-
ters not improbably owe their origin to much
the same causes. At least we do not know that
they do not. Concerning the real causes of
variations of any kind we know very little
more than we did when Darwin commented
on our profound ignorance of this subject. It
is therefore premature to pin our faith to any
particular theory of the origin of variation and

especially to draw far-reaching conclusions re-
garding evolution on the basis of such an in-
terpretation. We may conceive variability as
due to germinal losses or gains for the sake of
our formulas, and there may be little harm in
so doing so long as it is clearly realized that
the procedure is a purely arbitrary and
schematic method of recording certain facts
of inheritance. But when we make the serious
attempt to apply the conception to what actu-
ally takes place in the germ plasm we en-
counter a fruitful source of fallacies.

S. J. HOLMES

UNIVERSITY OF CALIFORNIA

3426G3

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