be rose (A) and absence of rose (a), and pea (B) and absence
of pea (b). The zygotic constitution of a rose is therefore
AAbb, and of a pea aaBB. A zygote containing both rose
and pea is a walnut: a zygote containing neither rose nor pea
is a single. The peculiar feature of such a case lies in the fact
that absence of rose and absence of pea are the same thing,
i.e. single; and this is doubtless owing to the fact that the
characters rose and pea both affect the same structure, the
comb.
3. Cases exist in which the characters due to one allelomorphic
pair can only become manifest in the presence of a
particular member of the other pair. If in fig. 3 the characters
due to B–b can only manifest themselves in the presence of
A, it is obvious that this can happen in twelve cases out of
sixteen, but not in the remaining four, which are homozygous
for aa. An example of this is to be found in the inheritance
of coat colour in rabbits, rats and mice where the allelomorphic
pairs concerned are wild grey colour (B) dominant to black
(b) and pigmentation (A) dominant to albinism (a). Certain
albinos (aaBB) crossed with blacks (AAbb) give only greys
(AaBb), and when these are bred together they give 9 greys,
3 blacks and 4 albinos. Of the 4 albinos 3 carry the grey
character and 1 does not, but in the absence of the pigmentation
factor (A) this is not visible. The ratio 9 : 3 : 4 must be
regarded as a 9 : 3 : 3 : 1 ratio, in which the last two terms are
visibly indistinguishable owing to the impossibility of telling
by the eye whether an albino carries the character for grey
or not.
4. The appearance of a zygotic character may depend upon
the coexistence in the zygote of two unit-characters belonging
to different allelomorphic pairs. If in the scheme shown in
fig. 3 the manifestation of a given character depends upon
the simultaneous presence of A and B, it is obvious that 9 of
the 16 zygotes will present this character, whilst the remaining
7 will be without it. This is shown graphically in fig. 5, where
the 9 squares have been shaded
and the 7 left plain. The sweet pea
offers an example of this phenomenon.
White sweet peas breed true
to whiteness, but when certain strains
of whites are crossed the offspring
are all coloured.. In the next generation
(F2) these F1 plants give rise to 9
coloured and 7 whites in every 16
plants. Colour here is a compound
character whose manifestation depends
upon the co-existence of two factors
in the zygote, and each of the original parents was homozygous
for one of the two factors necessary to the production
of colour. The ratio 9 : 7 is in reality a 9 : 3 : 3 : 1 ratio
in which, owing to special conditions; the zygotes represented
by the last three terms are indistinguishable from one another
by the eye.
AA
BB
AA
Bb
Aa
BB
Aa
Bb
AA
bB
AA
bb
Aa
bB
Aa
bb
aA
BB
aA
Bb
aa
BB
aa
Bb
aA
bB
aA
bb
aa
bB
aa
bb
Fig. 5.
The phenomena of dihybridism, as illustrated by the four
examples given above, have been worked out in many other
cases for plants and animals. Emphasis must be laid upon
the fact that, although the unit-characters belonging to two
pairs may react upon one another in the zygote and affect
its character, their inheritance is yet entirely independent.
Neither grey nor black can appear in the rabbit unless the
pigmentation factor is also present; nevertheless, gametic
segregation of this pair of characters takes place in the normal
way among albino rabbits, though its effects are never visible
until a suitable cross is made. In cases of trihybridism the
Mendelian ratio for the forms appearing in F2 is 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1, i.e. 27 showing dominance of three characters, three
groups of 9 each showing dominance of two characters, three
groups of 3 each showing dominance of one character, and
a single individual out of 64 which is homozygous for all three
recessive characters. It is obvious that the system can be
indefinitely extended to embrace any number of allelomorphic
pairs.
Reversion.—Facts such as those just dealt with in connexion
with certain cases of dihybridism throw an entirely new light
upon the phenomenon known as reversion on crossing. This
is now seen to consist in the meeting of factors which had in
some way or other become separated in phylogeny. The
albino rabbit when crossed with the, black “reverts” to the
wild grey colour, because each parent supplies one of the two
factors upon which the manifestation of the wild colour depends.
So also the wild purple sweet pea may come as a reversion
on crossing two whites. In such cases the reversion appears
in the F1 generation, because the two factors upon which it
depends are the dominants of their respective allelomorphic
pairs. Where the reversion depends upon the simultaneous
absence of two characters it cannot appear until the F2 generation.
When fowls with rose and pea combs are crossed the
reversionary single comb characteristic of the wild Gallus bankiva
first appears in the F2 generation.
CRB
CRB
CRB
CRb
CRb
CRB
CRb
CRb
CRB
cRB
CRB
cRb
CRb
cRB
CRb
cRb
CrB
CRB
CrB
CRb
Crb
CRB
Crb
CRb
CrB
cRB
CrB
cRb
Crb
cRB
Crb
cRb
CRB
CrB
CRB
Crb
CRb
CrB
CRb
Crb
CRB
crB
CRB
crb
CRb
crB
CRb
crb
CrB
CrB
CrB
Crb
Crb
CrB
Crb
Crb
CrB
crB
CrB
crb
Crb
crB
Crb
crb
cRB
CRB
cRB
CRb
cRb
CRB
cRb
CRb
cRB
cRB
cRB
cRb
cRb
cRB
cRb
cRb
crB
CRB
crB
CRb
crb
CRB
crb
CRb
crB
cRB
crB
cRb
crb
cRB
crb
cRb
cRB
CrB
cRB
Crb
cRb
CrB
cRb
Crb
cRB
crB
cRB
crb
cRb
crB
cRb
crb
crB
CrB
crB
Crb
crb
CrB
crb
Crb
crB
crB
crB
crb
crb
crB
crb
crb
Fig. 6.
Gametic Coupling.—In certain cases the distribution of characters
in heredity is complicated by the fact that particular
unit-characters tend to become associated or coupled together
during gametogenesis. In no case have we yet a complete
explanation of the phenomenon, but in view of the important
bearing which these facts must eventually have on our ideas
of the gametogenic process an illustration may be given.
The case in which two white sweet peas gave a coloured on
crossing has already been described, and it was seen that the
production of colour was dependent upon the meeting of two
factors, of which one was brought in by each parent. If the
allelomorphic pairs be denoted by C–c and R–r, then the zygotic
constitution of the two parents must have been CCrr and
ccRR respectively. The F1 plant may be either purple or red,
two characters which form an allelomorphic pair in which
the former is dominant, and which may be denoted by the
letters B–b. If B is brought in by one parent only the F1
plant will be heterozygous for all three allelomorphic pairs,
and therefore of the constitution Cc Rr Bb. In the F2 generation
the ratio of coloured to white must be 9 : 7, and of purple to
red 3 : 1; and experiment has shown that this generation is
composed on the average of 27 purples, 9 reds and 28 whites
out of every 64 plants. The exact composition of such a family
may be gathered from the accompanying table (fig. 6). So
far the case is perfectly smooth, and it is only on the introduction
of another character that the phenomenon of partial coupling
is witnessed. Two kinds of pollen grain occur in the sweet
pea. In some plants they are oblong in shape, whilst in others
they are round, the latter condition being recessive to the
former. If the original white parents were homozygous for long
and round respectively the F1 purple must be heterozygous, and
in the F2 generation, as experiment has shown, the ratio of
longs to rounds for the whole family is 3 : 1. But among the
purples there are about twelve longs to each round, the excess
of longs here being balanced by the reds, where the proportion
