SEX
[Illustration: FIG. 17.
_Abraxas grossulariata_, the common currant moth, and (on the right) its
paler lacticolor variety.]
In their simplest expression the phenomena exhibited by Mendelian
characters are sharp and clean cut. Clean cut and sharp also are the
phenomena of sex. It was natural, therefore, that a comparison should have
been early instituted between these two sets of phenomena. As a general
rule, the cross between a male and a female results in the production of
the two sexes in approximately equal numbers. The cross between a
heterozygous dominant and a recessive also leads to equal numbers of
recessives and of heterozygous dominants. Is it not, therefore, possible
that one of the sexes is heterozygous for a factor which is lacking in the
other, and that the presence or absence of this factor determines the sex
of the zygote? The results of some recent experiments would appear to
justify this interpretation, at any rate in particular cases. Of these, the
simplest is that of the common currant moth (_Abraxas grossulariata_), of
which there exists a pale variety (Fig. 17) known as _lacticolor_. The
experiments of Doncaster and Raynor showed that the variety behaved as a
simple recessive to the normal form. But the distribution of the dominants
and {100} recessives [Illustration]with with regard to the sexes was
peculiar. The original cross was between a _lacticolor_ female and a normal
male. All the F_1 moths of both sexes were of the normal _grossulariata_
type. The F_1 insects were then paired together and gave a generation
consisting of 3 normals : 1 _lacticolor_. But all the _lacticolor_ were
females, and all the males were of the normal pattern. It was, however,
found possible to obtain the _lacticolor male_ by mating a _lacticolor_
female with the F_1 male. The family resulting from this cross consisted of
normal males and normal females, _lacticolor_ males and _lacticolor_
females, and the {101} four sorts were produced in approximately equal
numbers. In such a family there was no special association of either of the
two colour varieties with one sex rather than the other. But the reverse
cross, F_1 female by _lacticolor_ male, gave a very different result. As in
the previous cross such families contained equal numbers of the normal form
and of the recessive variety. But all of the normal _grossulariata_ were
males, while all the _lacticolor_ were females. Now this seemingly complex
collection of facts is readily explained if we make the following three
assumptions:--
[Illustration]
(1) The _grossulariata_ character (G) is dominant to the lacticolor
character (g). This is obviously justified by the experiments, for, leaving
the sex distribution out of account, we get the expected 3 : 1 ratio from
F_1 x F_1, and also the expected ratio of equality when the heterozygote is
crossed with the recessive.
(2) The female is heterozygous for a dominant factor (F) which is lacking
in the male. The constitution of a female is consequently Ff, and of a male
ff. This assumption is in harmony with the fact that the sexes are produced
in approximately equal numbers.
(3) There exists repulsion between the factors G and F in a zygote which is
heterozygous for them both. Such zygotes (FfGg) must always be females, and
on this assumption will produce gametes Fg and fG in equal numbers. {102}
[Illustration: FIG. 18.
Scheme of inheritance in the F_1 and F_2 generations resulting from the
cross of _lacticolor_ female with _grossulariata_ male. The character of
each individual is represented by the sex signs in brackets, the black
being _grossulariata_ in appearance and the light ones _lacticolor_.]
We may now construct a scheme for comparison with that on page 100 to show
how these assumptions explain the experimental results. The original
parents were _lacticolor_ female and _grossulariata_ male, which on our
assumptions must be Ffgg and ffGG respectively in constitution. Since the
female is always heterozygous for F, her gametes must be of two kinds, viz.
Fg and fg, while those of the pure _grossulariata_ male must be all fG.
When an ovum Fg is fertilised by a spermatozoon fG, the resulting zygote,
FfGg, is heterozygous for both F and G, and in appearance is a female
_grossulariata_. The zygote resulting from the fertilisation of an ovum fg
by a spermatozoon fG is heterozygous for G, but does not contain F, and
therefore is a male _grossulariata_. Such a male being in constitution
{103} ffGg must produce gametes of two kinds, fG and fg, in equal numbers.
And since we are assuming repulsion between F and G, the F_1 female being
in constitution FfGg, must produce equal numbers of gametes Fg and fG. For
on our assumption F and G cannot enter into the same gamete. The series of
gametes produced by the F_1 moths, therefore, are fG, fg by the male and
Fg, fG by the female. The resulting F_2 generation consequently consists of
the four classes of zygotes Ffgg, FfGg, ffGg, and ffGG in equal numbers. In
other words, the sexes are produced in equal numbers, the proportion of
normal grossulariata to _lacticolor_ is 3 : 1, and all of the _lacticolor_
are females; that is to say, the results worked out on our assumptions
accord with those actually produced by experiment. We may now turn to the
results which should be obtained by crossing the F_1 moths with the
_lacticolor_ variety. And first we will take the cross _lacticolor_ female
x F_1 male. The gametes produced by the lacticolor female we have already
seen to be Fg and fg, while those produced by the F_1 male are fG and fg.
The bringing together of these two series of gametes must result in equal
numbers of the four kinds of zygotes FfGg, Ffgg, ffGg, and ffgg, _i.e._ of
female _grossulariata_ and _lacticolor_, and of male _grossulariata_ and
_lacticolor_ in equal numbers. Here, again, the calculated results accord
with those of experiment. Lastly, we may examine what should happen when
the F_1 female is crossed with the _lacticolor_ {104} male. The F_1 female,
owing to the repulsion between F and G, produces only the two kinds of ova
Fg and fG, and produces them in equal numbers. Since the _lacticolor_ male
can contain neither F nor G, all of its spermatozoa must be fg. The results
of such a cross, therefore, should be to produce equal numbers of the two
kinds of zygote Ffgg and ffGg, _i.e._ of _lacticolor_ females and of
_grossulariata_ males. And this, as we have already seen, is the actual
result of such a cross.
Before leaving the currant moth we may allude to an interesting discovery
which arose out of these experiments. The _lacticolor_ variety in Great
Britain is a southern form and is not known to occur in Scotland. Matings
were made between wild Scotch females and _lacticolor_ males. The families
resulting from such matings were precisely the same as those from
_lacticolor_ males and F_1 females, viz. _grossulariata_ males and
_lacticolor_ females only. We are, therefore, forced to regard the
constitution of the wild _grossulariata_ female as identical with that of
the F_1 female, _i.e._ as heterozygous for the _grossulariata_ factor as
well as for the factor for femaleness. Though from a region where
_lacticolor_ is unknown, the "pure" wild _grossulariata_ female is
nevertheless a permanent mongrel, but it can never reveal its true colours
unless it is mated with a male which is either heterozygous for G or pure
_lacticolor_. And as all the wild northern males are {105} pure for the
_grossulariata_ character this can never happen in a state of nature.
[Illustration: FIG. 19.
Scheme illustrating the result of crossing a Silky hen with a Brown Leghorn
cock. Black sex signs denote deeply pigmented birds, and light sex signs
those without pigmentation. The light signs with a black dot in the centre
denote birds with a small amount of pigment.]
An essential feature of the case of the currant moth lies in the different
results given by reciprocal crosses. _Lacticolor_ female x _grossulariata_
male gives _grossulariata_ alone of both sexes. But _grossulariata_ female
x _lacticolor_ male gives only _grossulariata_ males and _lacticolor_
females. Such a difference between reciprocal crosses has also been found
in other animals, and the experimental results, though sometimes more
complicated, are explicable on the same lines. An interesting case in which
three factors are concerned has been recently worked out in poultry. The
Silky breed of fowls is characterised among other peculiarities by a
remarkable abundance of melanic pigment. The skin is dull black, while the
comb and wattles are of a deep purple colour contrasting sharply with the
white plumage (Pl. V., 3). Dissection shows that this black pigment is
widely spread throughout the body, being especially marked in such
membranes as the mesenteries, the periosteum, and the pia mater surrounding
the brain. It also occurs in the connective tissues among the muscles. In
the Brown Leghorn, on the other hand, this pigment is not found. Reciprocal
crosses between these two breeds gave a remarkable difference in result. A
cross between the Silky hen and the Brown Leghorn cock produced F_1 birds
in which both sexes exhibited only traces of the pigment. On casual
observation they might have {106} passed for unpigmented birds, for with
the exception of an occasional fleck of pigment their skin, comb and
wattles were as clear as in the Brown Leghorn (Pl. V., 1 and 4). Dissection
revealed the presence of a slight amount of internal pigment. Such birds
bred together gave some offspring with the full pigmentation of the Silky,
some without any pigment, and others showing different degrees of pigment.
None of the F_2 male birds, however, showed the full deep pigmentation of
the Silky.
[Illustration: FIG. 20.
Scheme illustrating the result of crossing a Brown Leghorn hen with a Silky
cock (cf. Fig. 19).]
When, however, the cross was made the other way, viz. Brown Leghorn hen x
Silky cock, the result was different. While the F_1 male birds were almost
destitute of pigment as in the previous cross, the F_1 hens, on the other
hand, were nearly as deeply pigmented as the pure Silky {107} (Pl. V., 2).
The male Silky transmitted the pigmentation, but only to his daughters.
Such birds bred together gave an F_2 generation containing chicks with the
full deep pigment, chicks without pigment, and chicks with various grades
of pigmentation, all the different kinds in both sexes.
[Illustration: FIG. 21.
Scheme to illustrate the result of crossing F_1 birds (_e.g._ Brown Leghorn
x Silky) with the pure Brown Leghorn.]
In analysing this complicated case many other different crosses were made,
but for the present it will be sufficient to mention but one of these, viz.
that between the F_1 birds and the pure Brown Leghorn. The cross between
the F_1 hen and the Brown Leghorn cock produced only birds with a slight
amount of pigment and birds without pigment. And this was true for both the
deeply pigmented and the slightly pigmented types of F_1 hen. But when the
F_1 cock was mated to a Brown Leghorn hen, a definite proportion of the
chicks, one in eight, was deeply pigmented, and _these deeply pigmented
birds were always females_ (cf. Fig. 21). And in this respect all the F_1
males behaved alike, whether they were from the Silky hen or from the Silky
cock. We have, therefore, the paradox that the F_1 hen, though herself
deeply pigmented, cannot transmit this condition to any of her offspring
when she is mated to the unpigmented Brown Leghorn, but that, when
similarly mated, the F_1 cock can transmit this pigmented condition to a
quarter of his female offspring though he himself is almost devoid of
pigment.
[Illustration: PLATE V.
1, 2, F_1 Cock and Hen, ex Brown Leghorn Hen x Silky Cock; 3, Silky Cock;
4, Hen ex Silky Hen x Brown Leghorn Cock.]
{108}
[Illustration: FIG. 22.
Scheme to illustrate the nature of the F_1 generation from the Silky hen
and Brown Leghorn cock (cf. Fig. 23).]
Now all these apparently complicated results, as well as many others to
which we have not alluded, can be expressed by the following simple scheme.
There are three factors affecting pigment, viz. (1) a pigmentation factor
(P); (2) a factor which inhibits the production of pigment (I); and (3) a
factor for femaleness (F), for which the female birds are heterozygous, but
which is not present in the males. Further, we make the assumptions (a)
that there is repulsion between F and I in the female zygote (FfIi), and
(b) that the male Brown Leghorn is homozygous for the inhibitor factor (I),
but that the hen Brown Leghorn is always heterozygous for this factor just
in the same way as the female of the currant moth is always heterozygous
for the _grossulariata_ factor. We may now proceed to show how this
explanation fits the experimental facts which we have given.
The Silky is pure for the pigmentation factor, but does not contain the
inhibitor factor. The Brown Leghorn, on the other hand, contains the
inhibitor factor, but not the {109} pigmentation factor. In crossing a
Silky hen with a Brown Leghorn cock we are mating two birds of the
constitution FfPPii and ffppII, and all the F_1 birds are consequently
heterozygous for both P and I. In such birds the pigment is almost but not
completely suppressed, and as both sexes are of the same constitution with
regard to these two factors they are both of similar appearance.
[Illustration: FIG. 23.
Scheme to illustrate the nature of the F_1 generation from the Brown
Leghorn hen and Silky cock (cf. Fig. 22).]
In the reciprocal cross, on the other hand, we are mating a Silky male
(ffPPii) with a Brown Leghorn hen which on our assumption is heterozygous
for the inhibitor factor (I), and in constitution therefore is FfppIi.
Owing to the repulsion between F and I the gametes produced by such a bird
are Fpi and fpI in equal numbers. All the gametes produced by the Silky
cock are fPi. Hence the constitution of the F_1 male birds produced by this
cross is ffPpIi as before, but the female birds must be all of the
constitution FfPpii. The Silky cock transmits the fully pigmented condition
to his daughters, because the gametes of the Brown Leghorn hen which
contain the factor for femaleness do not contain the {110} inhibitory
factor owing to the repulsion between these factors. The nature of the F_2
generation in each case is in harmony with the above scheme. As, however,
it serves to illustrate certain points in connection with intermediate
forms we shall postpone further consideration of it till we discuss these
matters, and for the present shall limit ourselves to the explanation of
the different behaviour of the F_1 males and females when crossed with the
Brown Leghorn. And, first, the cross of Brown Leghorn female by F_1 male.
The Brown Leghorn hen is on our hypothesis FfppIi, and produces gametes Fpi
and fpI. The F_1 cock is on our hypothesis ffPpIi, and produces in equal
numbers the four kinds of gametes fPI, fPi, fpI, fpi. The result of the
meeting of these two series of gametes is given in Fig. 24. Of the eight
different kinds of zygote formed only one contains P in the absence of I,
and this is a female. The result, as we have already seen, is in accordance
with the experimental facts.
[Illustration: FIG. 24.
Diagram showing the nature of the offspring from a Brown Leghorn hen and an
F_1 cock bred from Silky hen x Brown Leghorn cock, or _vice versa_.]
On the other hand, the Brown Leghorn cock is on our hypothesis ffppII. All
his gametes consequently contain the inhibitor factor, and when he is mated
with an F_1 {111} hen all the zygotes produced must contain I. None of his
offspring, therefore, can be fully pigmented, for this condition only
occurs in the absence of the inhibitor factor among zygotes which are
either homozygous or heterozygous for P.
[Illustration: FIG. 25.
Scheme to illustrate the heterozygous nature of the pure Brown Leghorn hen.
For explanation see text.]
The interpretation of this case turns upon the constitution of the Brown
Leghorn hen, upon her heterozygous condition with regard to the two factors
F and I, and upon the repulsion that occurs between them when the gametes
are formed. Through an independent set of experiments this view of the
nature of the Brown Leghorn hen has been confirmed in an interesting way.
There are fowls which possess neither the factor for pigment nor the
inhibitory factor, which are in constitution ppii. Such birds when crossed
with the Silky give dark pigmented birds of both sexes in F_1, and the F_2
generation consists of pigmented and unpigmented in the ratio 3 : 1. Now a
cock of such a strain crossed with a Brown Leghorn hen should give only
completely unpigmented birds. But if, as we have supposed, the Brown
Leghorn hen is producing gametes Fpi and fpI, the male birds produced by
such a cross should be heterozygous for I, {112} _i.e._ in constitution
ffppIi, while the hen birds, though identical in appearance so far as
absence of pigmentation goes, should not contain this factor but should be
constitutionally Ffppii. Crossed with the pure Silky, the F_1 birds of
opposite sexes should give an entirely different result. For while the hens
should give only deeply pigmented birds of both sexes, the cocks should
give equal numbers of deeply pigmented and slightly pigmented birds (cf.
Fig. 25). These were the results which the experiment actually gave, thus
affording strong confirmation of the view which we have been led to take of
the Brown Leghorn hen. Essentially the poultry case is that of the currant
moth. It differs in that the factor which {113} repels femaleness produces
no visible effect, and its presence or absence can only be determined by
the introduction of a third factor, that for pigmentation.
This conception of the nature of the Brown Leghorn hen leads to a curious
paradox. We have stated that the Silky cock transmits the pigmented
condition, but transmits it to his daughters only. Apparently the case is
one of unequal transmission by the father. Actually, as our analysis has
shown, it is one of unequal transmission by the mother, the father's
contribution to the offspring being identical for each sex. The mother
transmits to the daughters her dominant quality of femaleness, but to
balance this, as it were, she transmits to her sons another quality which
her daughters do not receive. It is a matter of common experience among
human families that in respect to particular qualities the sons tend to
resemble their mothers more than the daughters do, and it is not improbable
that such observations have a real foundation for which the clue may be
provided by the Brown Leghorn hen.
Nor is this the only reflection that the Brown Leghorn suggests. Owing to
the repulsion between the factors for femaleness and for pigment
inhibition, it is impossible by any form of mating to make a hen which is
homozygous for the inhibitor factor. She has bartered away for femaleness
the possibility of ever receiving a double dose of this factor. We know
that in some cases, as, for example, {114} that of the blue Andalusian
fowl, the qualities of the individual are markedly different according as
to whether he or she has received a single or a double dose of a given
factor. It is not inconceivable that some of the qualities in which a man
differs from a woman are founded upon a distinction of this nature. Certain
qualities of intellect, for example, may depend upon the existence in the
individual of a double dose of some factor which is repelled by femaleness.
If this is so, and if woman is bent upon achieving the results which such
qualities of intellect imply, it is not education or training that will
help her. Her problem is to get the factor on which the quality depends
into an ovum that carries also the factor for femaleness.
* * * * *
{115}