DOMINANCE
[Illustration: FIG. 12.
Primula flowers to illustrate the intermediate nature of the F_1 flower
when _sinensis_ is crossed with _stellata_.]
Sinensis x Stellata
|
Intermediate -------------------------- F_1
|
+---------+----+-------+-----------+
Sinensis Inter. Inter. Stellata --------- F_2
| | |
Sinensis | Stellata --------- F{3}
| +-----+-----+-----+ |
Sinensis Sin. Int. Int. Stell. Stellata --------- F{4}
In the cases which we have hitherto considered the presence of a factor
produces its full effect whether it is introduced by both of the gametes
which go to form the zygote, or by one of them alone. The heterozygous tall
pea or the heterozygous rose-combed fowl cannot be distinguished from the
homozygous form by mere inspection, however close. Breeding tests alone can
decide which is the heterozygous and which the homozygous form. Though this
is true for the majority of characters yet investigated, there are cases
known in which the heterozygous form differs in appearance from either
parent. Among plants such a case has been met with in the primula. The
ordinary Chinese primula (_P. sinensis_) (Fig. 12) has large rather wavy
petals much crenated at the edges. In the Star Primula (_P. stellata_) the
flowers are much smaller, while the petals are flat and present only a
terminal notch instead of the numerous crenations of _P. sinensis_. The
heterozygote produced by crossing these forms is intermediate in size and
appearance. When self-fertilised such plants behave in simple Mendelian
fashion, {69} giving a generation consisting of _sinensis_, intermediates,
and _stellata_ in the ratio 1 : 2 : 1. Subsequent breeding from these
plants showed that both the _sinensis_ and _stellata_ which appeared in the
F_2 generation bred true, while the intermediates always gave all three
forms again in the same proportion. But though there is no dominance of the
character of either parent in such a case as this, the Mendelian principle
of segregation could hardly have a better illustration.
{70}
Blue x Blue
|
+-------------+--------+------------+
Black Blue x Blue White
| | |
| +------+--+--+-----+ |
Black Black Blue Blue White White
| |
Black ------------- x ------------- White
|
Blue
(all)
Among birds a case of similar nature is that of the Blue Andalusian fowl.
Fanciers have long recognised the difficulty of getting this variety to
breed true. Of a slaty blue colour itself with darker hackles and with
black lacing on the feathers of the breast, it always throws "wasters" of
two kinds, viz. blacks, and whites splashed with black. Careful breeding
from the blues shows that the three sorts are always produced in the same
definite {71} proportions, viz., one black, two blues, one splashed white.
This at once suggests that the black and the splashed white are the two
homozygous forms, and that the blues are heterozygous, _i.e._, producing
equal numbers of "black" and "white splashed" gametes. The view was tested
by breeding the "wasters" together--black with black, and splashed white
with splashed white--and it was found that each bred true to its respective
type. But when the black and the splashed white were crossed they gave, as
was expected, nothing but blues. In other words, we have the seeming
paradox of the black and the splashed white producing twice as many blues
as do the blues when bred together. The black and the splashed white
"wasters" are in reality the pure breeds, while the "pure" Blue Andalusian
is a mongrel which no amount of selection will ever be able to fix.
In such cases as this it is obvious that we cannot speak of dominance. And
with the disappearance of this phenomenon we lose one criterion for
determining which of the two parent forms possesses the additional factor.
Are we, for example, to regard the black Andalusian as a splashed white to
which has been added a double dose of a colour-intensifying factor, or are
we to consider the white splashed bird as a black which is unable to show
its true pigmentation owing to the possession of some inhibiting factor
which prevents the manifestation of the black. Either interpretation fits
the facts equally well, {72} and until further experiments have been
devised and carried out it is not possible to decide which is the correct
view.
Besides these comparatively rare cases where the heterozygote cannot be
said to bear a closer resemblance to one parent more than to the other,
there are cases in which it is often possible to draw a visible distinction
between the heterozygote and the pure dominant. There are certain white
breeds of poultry, notably the White Leghorn, in which the white behaves as
a dominant to colour. But the heterozygous whites made by crossing the
dominant white birds with a pure coloured form (such as the Brown Leghorn)
almost invariably show a few coloured feathers or "ticks" in their plumage.
The dominance of white is not quite complete, and renders it possible to
distinguish the pure from the impure dominant without recourse to breeding
experiments.
+------+------+------+------+
|CI |CI |CI |CI |
|CI |Ci |cI |ci |
| | | | |
| | | | |
+------+------+------+------+
|Ci |Ci....|Ci |Ci....|
|CI |Ci....|cI |ci....|
| |......| |......|
| |......| |......|
+------+------+------+------+
|cI |cI |cI |cI |
|CI |Ci |cI |ci |
| | | | |
| | | | |
+------+------+------+------+
|ci |ci....|ci |ci |
|CI |Ci....|cI |ci |
| |......| | |
| |......| | |
+------+------+------+------+
FIG. 13.
Diagram to illustrate the nature of the F_2 generation from the cross
between dominant white and recessive white fowls.
This case of the dominant white fowl opens up another interesting problem
in connection with dominance. By accepting the "Presence and Absence"
hypothesis we are committed to the view that the dominant form possesses an
extra factor as compared with the recessive. The natural way of looking at
this case of the fowl is to regard white as the absence of colour. But were
this so, colour should be dominant to white, which is not the case. We are
therefore forced to suppose that the absence of colour in this instance is
due to the presence of a factor whose {73} property is to inhibit the
production of colour in what would otherwise be a pure coloured bird. On
this view the dominant white fowl is a coloured bird plus a factor which
inhibits the development of the colour. The view can be put to the test of
experiment. We have already seen that there are other white fowls in which
white is recessive to colour, and that the whiteness of such birds is due
to the fact that they lack a factor for the development of colour. If we
denote this factor by C and our postulated inhibitor factor in the dominant
white bird by I, then we must write the constitution of the recessive white
as ccii, and the dominant white as CCII. We may now work out the results we
ought to obtain when a cross is made between these two pure white breeds.
The constitution of the F_1 bird must be CcIi. Such birds being
heterozygous for the inhibitor factor, should be whites showing some
coloured "ticks." Being heterozygous for both of the two factors C and I,
they will produce in equal numbers the four different sorts of gametes CI,
Ci, cI, ci. The result of bringing two such similar series of gametes
together is shown in Fig. 13. Out of the sixteen squares, twelve contain I;
these will be white birds either with or without a few coloured ticks.
Three contain C but not I: these must be coloured birds. One contains
neither C nor I; this must be a white. From such a mating we ought,
therefore, to obtain both white and coloured birds in the ratio 13 : 3. The
results thus theoretically {74} deduced were found to accord with the
actual facts of experiment. The F_1 birds were all "ticked" whites, and in
the F_2 generation came white and coloured birds in the expected ratio.
There seems, therefore, little reason to doubt that the dominant white is a
coloured bird in which the absence of colour is due to the action of a
colour-inhibiting factor, though as to the nature of that factor we can at
present make no surmise. It is probable that other facts, which at first
sight do not appear to be in agreement with the "Presence and Absence"
hypothesis, will eventually be brought into line through the action of
inhibitor factors. Such a case, for instance, is that of bearded and
beardless wheats. Though the beard is obviously the additional character,
the bearded condition is recessive to the beardless. Probably we ought to
regard the beardless as a bearded wheat in which there is an inhibitor that
stops the beard from growing. It is not unlikely that as time goes on we
shall {75} find many more such cases of the action of inhibitor factors,
and we must be prepared to find that the same visible effect may be
produced either by the addition or by the omission of a factor. The
dominant and recessive white poultry are indistinguishable in appearance.
Yet the one contains a factor more and the other a factor less than the
coloured bird.
[Illustration: FIG. 14.
Ears of beardless and bearded wheat. The beardless condition is dominant to
the bearded.]
{76}
A phenomenon sometimes termed irregularity of dominance has been
investigated in a few cases. In certain breeds of poultry such as Dorkings
there occurs an extra toe directed backwards like the hallux (cf. Fig. 15).
In some families this character behaves as an ordinary dominant to the
normal, giving the expected 3 : 1 ratio in F_2. But in other families
similarly bred the proportions of birds with and without the extra toe
appear to be unusual. It has been shown that in such a family some of the
birds without the extra toe may nevertheless transmit the peculiarity when
mated with birds belonging to strains in which the extra toe never occurs.
Though the external appearance of the bird generally affords some
indication of the nature of the gametes which it is carrying, this is not
always the case. Nevertheless we have reason to suppose that the character
segregates in the gametes, though the nature of these cannot always be
decided from the appearance of the bird which bears them.
[Illustration: FIG. 15.
Fowls' feet. On the right a normal and on the left one with an extra toe.]
[Illustration: FIG. 16.
Scheme to illustrate the inheritance of horns in sheep. Heterozygous males
shown dark with a white spot, heterozygous females light with a dark spot
in the centre.]
There are cases in which an apparent irregularity of dominance has been
shown to depend upon another character, as in the experiments with sheep
carried out by Professor Wood. In these experiments two breeds were
crossed, of which one, the Dorset, is horned in both sexes, while the
other, the Suffolk, is without horns in either sex. Whichever way the cross
was made the resulting F_1 generation was similar; the rams were horned,
and {77} the ewes were hornless. In the F_2 generation raised from these
F_1 animals both horned and hornless types appeared in both sexes but in
very different proportions. While the horned rams were about three times as
numerous as the hornless, this relation was reversed among the females, in
which the horned formed only about one-quarter of the total. The simplest
explanation of this interesting case is to suppose that the dominance of
the horned character depends upon the sex of the animal--that it is
dominant in the male but recessive in the female. A pretty experiment was
devised for putting this view to the test. If it is true, equal numbers of
gametes with and without the horned factor must be produced by the F_1
ewes, while the factor should be lacking in all the gametes of the hornless
F_2 rams. A {78} hornless ram, therefore, put to a flock of F_1 ewes should
give rise to equal numbers of zygotes which are heterozygous for the horned
character, and of zygotes in which it is completely absent. And since the
heterozygous males are horned, while the heterozgyous females are hornless,
we should expect from this mating equal numbers of horned and hornless
rams, but only hornless ewes. The result of the experiment confirmed this
expectation. Of the ram lambs 9 were horned and 8 were hornless, while all
the 11 ewe lambs were completely destitute of horns.
[Illustration: PLATE III.
Sheep]
* * * * *
{79}