SEX (_continued_)
The cases which we have considered in the last chapter belong to a group in
which the peculiarities of inheritance are most easily explained by
supposing that the female is heterozygous for some factor that is not found
in the male. Femaleness is an additional character superposed upon a basis
of maleness, and as we imagine that there is a separate factor for each the
full constitutional formula for a female is FfMM, and for a male ffMM. Both
sexes are homozygous for the male element, and the difference between them
is due to the presence or absence of the female element F.
There are, however, other cases for which the explanation will not suffice,
but can be best interpreted on the view that the male is heterozygous for a
factor which is not found in the female. Such a case is that recently
described by Morgan in America for the pomace fly (_Drosophila
ampelophila_). Normally this little insect has a red eye, but white eyed
individuals are known to occur as rare sports. Red eye is dominant to
white. In their relation to sex the eye colours of the pomace fly {116} are
inherited on the same lines as the _grossulariata_ and _lacticolor_
patterns of the currant moth, but with one essential difference. The factor
which repels the red-eye factor is in this case to be found in the male,
and here consequently it is the male which must be regarded as heterozygous
for a sex factor that is lacking in the female.
[Illustration]
In order to bring these cases and others into line an interesting
suggestion has recently been put forward by Bateson. On this suggestion
each sex is heterozygous for its own sex factor only, and does not contain
the factor proper to the opposite sex. The male is of the constitution,
Mmff and the female Ffmm. Each sex produces two sorts of gametes, Mf and mf
in the case of the male, and Fm, fm in that of the female. But on this view
a further supposition is necessary. If each of the two kinds of spermatozoa
were capable of fertilising each of the two kinds of ova, we should get
individuals of the constitution MmFf and mmff, as well as the normal males
and females, Mmff and Ffmm. As the facts of ordinary bisexual reproduction
afford us no grounds for assuming the existence of these two classes of
individuals, whatever they may be, we must suppose that fertilisation. is
productive only between the spermatozoa carrying M and the ova without F,
or between the spermatozoa {117} without M and the ova containing F. In
other words we must on this view suppose that fertilisations between
certain forms of gametes, even if they can occur, are incapable of giving
rise to zygotes with the capacity for further development. If we admit this
supposition, the scheme just given will cover such cases as those of the
currant moth and the fowl, equally as well as that of the pomace fly. In
the former there is repulsion between either the _grossulariata_ factor and
F, or else between the pigment inhibitor factor and F, while in the latter
there is repulsion between the factor for red eye and M.
[Illustration: FIG. 26.
Scheme to illustrate the probable mode of inheritance of colour-blindness.
The dark signs represent affected individuals. A black dot in the centre
denotes an unaffected female who is capable of transmitting the condition
to her sons.]
Whatever the merits or demerits of such a scheme it certainly does offer an
explanation of a peculiar form of sex limited inheritance in man. It has
long been a matter of common knowledge that colour-blindness is much more
common among men than among women, and also that unaffected women can
transmit it to their sons. At first sight the case is not unlike that of
the sheep, where the horned character is apparently dominant in the male
but recessive in the female. The hypothesis that the colour-blind condition
is due to the presence of an extra factor as compared with the normal, and
that a single dose of it will produce {118} colour-blindness in the male
but not in the female, will cover a good many of the observed facts (cf.
Fig. 26). Moreover, it serves to explain the remarkable fact that _all_ the
sons of colour-blind women are also colour-blind. For a woman cannot be
colour-blind unless she is homozygous for the colour-blind factor, in which
case all her children must get a single dose of it even if she marries a
normal male. And this is sufficient to produce colour-blindness in the
male, though not in the female.
But there is one notable difference in this case as compared with that of
the sheep. When crossed with pure hornless ewes the heterozygous horned ram
transmits the horned character to half his male offspring (cf. p. 71). But
the heterozygous colour-blind man does not behave altogether like a sheep,
for he apparently does not transmit the colour-blind condition to any of
his male offspring. If, however, we suppose that the colour-blind factor is
repelled by the factor for maleness, the amended scheme will cover the
observed facts. For, denoting the colour-blind factor by X, the gametes
produced by the colour-blind male are of two sorts only, viz. Mfx and mfX.
If he marries a normal woman (Ffmmxx), the spermatozoa Mfx unite with ova
fmx to give normal males, while the spermatozoa mfX unite with ova Fmx to
give females which are heterozygous for the colour-blind factor. These
daughters are themselves normal, but transmit the condition to about half
their sons. {119}
The attempt to discover a simple explanation of the nature of sex has led
us to assume that certain combinations between gametes are incapable of
giving rise to zygotes which can develop further. In the various cases
hitherto considered there is no reason to suppose that anything of the sort
occurs, or that the different gametes are otherwise than completely fertile
one with another. One peculiar case, however, has been known for several
years in which some of the gametes are apparently incapable of uniting to
produce offspring. Yellow in the mouse is dominant to agouti, but hitherto
a homozygous yellow has never been met with. The yellows from families
where only yellows and agoutis occur produce, when bred together, yellows
and agoutis in the ratio 2 : 1. If it were an ordinary Mendelian case the
ratio should be 3 : 1, and one out of every three yellows so bred should be
homozygous and give only yellows when crossed with agouti. But Cuenot and
others have shown that _all_ of the yellows are heterozygous, and when
crossed with agoutis give both yellows and agoutis. We are led, therefore,
to suppose that an ovum carrying the yellow factor is unproductive if
fertilised by a spermatozoon which also bears this factor. In this way
alone does it seem possible to explain the deficiency of yellows and the
absence of homozygous ones in the families arising from the mating of
yellows together. At present, however, it remains the only definite
instance among animals in which we have {120} grounds for assuming that
anything in the nature of unproductive fertilisation takes place.[8]
If we turn from animals to plants we find a more complicated state of
affairs. Generally speaking, the higher plants are hermaphrodite, both
ovules and pollen grains occurring on the same flower. Some plants, however
like most animals, are of separate sexes, a single plant bearing only male
or female flowers. In other plants the separate flowers are either male or
female, though both are borne on the same individual. In others, again, the
conditions are even more complex, for the same plant may bear flowers of
three kinds, viz. male, female, and hermaphrodite. Or it may be that these
three forms occur in the same species but in different individuals--female
and hermaphrodites in one species; males, females, and hermaphrodites in
another. One case, however, must be mentioned as it suggests a possibility
which we have not hitherto encountered. In the common English bryony
(_Bryonia dioica_) the sexes are separate, some plants having only male and
others only female flowers. In another European species, _B. alba_, both
male and female flowers occur on the same plant. Correns crossed these two
species reciprocally, and also fertilised _B. dioica_ by its own male with
the following results:--
{121}
dioica [female] x dioica [male] gave [female] [female] and [male] [male]
" x alba [male] " [female] [female] only
alba [female] x dioica [male] " [female] [female] and [male] [male].
The point of chief interest lies in the striking difference shown by the
reciprocal crosses between _dioica_ and _alba_. Males appear when _alba_ is
used as the female parent but not when the female _dioica_ is crossed by
male _alba_. It is possible to suggest more than one scheme to cover these
facts, but we may confine ourselves here to that which seems most in accord
with the general trend of other cases. We will suppose that in _dioica_
femaleness is dominant to maleness, and that the female is heterozygous for
this additional factor. In this species, then, the female produces equal
numbers of ovules with and without the female factor, while this factor is
absent in all the pollen grains. _Alba_ [female] x _dioica_ [male] gives
the same result as _dioica_ [female] x _dioica_ [male], and we must
therefore suppose that alba produces male and female ovules in equal
numbers. _Alba_ [male] x _dioica_ [female], however, gives nothing but
females. Unless, therefore, we assume that there is selective fertilisation
we must suppose that all the pollen grains of alba carry the female
factor--in other words, that so far as the sex factors are concerned there
is a difference between the ovules and pollen grains borne by the same
plant. Unfortunately further investigation of this case is rendered
impossible owing to the complete sterility of the F_1 plants. {122}
[Illustration: FIG. 27.
Single and double stocks raised from the same single parent.]
That the possibility of a difference between the ovules and pollen grains
of the same individual must be taken into account in future work there is
evidence from quite a different source. The double stock is an old
horticultural favourite, and for centuries it has been known that of itself
it sets no seed, but must be raised from special strains of the single
variety. "You must understand withall," wrote John Parkinson of his
gilloflowers,[9] "that those plants that beare double flowers, doe beare no
seed at all ... but the onely way to have double flowers any yeare is to
save the seedes of those plants of this kinde that beare single flowers,
for from that seede will rise some that will beare single, and some double
flowers." With regard to the nature of these double-throwing strains of
singles, Miss Saunders has recently brought out some interesting facts. She
crossed the double-throwing singles with pure singles belonging to strains
in which doubles never occur. The cross was made both ways, and in both
cases all the F_1 plants were single. A distinction, however, appeared when
a further generation was raised from the F_1 plants. All the F_1 plants
from the pollen of the double-throwing single behaved like double-throwing
singles, but of the F_1 plants from the ovules of the double throwers some
behaved as double throwers, and some as pure singles. We are led to infer,
therefore, that the ovules and pollen grains {123} of the double throwers,
though both produced by the same plant, differ in their relation to the
factor (or factors) for doubleness. Doubleness is apparently carried by all
the pollen grains of such plants, but only by some of the ovules. Though
the nature of doubleness in stocks is not yet clearly understood, the facts
discovered by Miss Saunders suggest strongly that the ovules and pollen
grains of the same plant may differ in their transmitting properties,
probably owing to some process of segregation in the growing plant which
leads to an unequal distribution of some or other factors to the cells
which give rise to the ovules as compared with those from which {124} the
pollen grains eventually spring. Whether this may turn out to be the true
account or not, the possibility must not be overlooked in future work.
Single
|
+-------------+
Single Double
/ \
Pollen of x Ovule Pollen x Ovule of
pure single | | pure single
| |
+----------+ |
Single Single Single
| | |
| +-------+ +---------+
Single Single Double Single Double
| | |
| +-------+ +---------+
Single Single Double Single Double
From all this it is clear enough that there is much to be done before the
problem of sex is solved even so far as the biologist can ever expect to
solve it. The possibilities are many, and many a fresh set of facts is
needed before we can hope to decide among them. Yet the occasional glimpses
of clear-cut and orderly phenomena, which Mendelian spectacles have already
enabled us to catch, offer a fair hope that some day they may all be
brought into focus, and assigned their proper places in a general scheme
which shall embrace them all. Then, though not till then, will the problem
of the nature of sex pass from the hands of the biologist into those of the
physicist and the chemist.
* * * * *
{125}