B. Absence of dominance, the heterozygote being more or less
intermediate in form.
Black and white splashed plumage (Andalusian fowls).
Lax and dense ears (wheat).
Six rowed and two rowed ears (barley).
Dominance.—The meaning of this phenomenon is at present
obscure, and we can make no, suggestion as to why it should
be complete in one case, partial in another, and entirely absent
in a third. When found it is as a rule definite and orderly,
but there are cases known where irregularity exists. The
extra toe characteristic of certain breeds of fowls, such as Dorkings,
behaves generally as a dominant character, but in certain
cases it has been ascertained that a fowl without an extra toe
may yet carry the extra toe character. It is possible that in
some cases dominance may be conditioned by the presence of
other features, and certain crosses in sheep lend colour to the
supposition that sex may be such a feature. A cross between
the polled Suffolk and the horned Dorset breeds results in
horned rams and polled ewes only, though in the F2 generation
both sexes appear with and without horns. At present the
simplest hypothesis which fits the facts is that horns are dominant
in the male and recessive in the female. It is important
not to confuse cases of apparent reversal of dominance such
as the above with cases in which a given visible character may
be the result of two entirely different causes. One white hen
may give only colour chicks by a coloured cock, whilst the
same cock with another white hen, indistinguishable in appearance
from the former, will give only white chickens containing
a few dark ticks. There is here no reversal of dominance,
but, as has been abundantly proved by experiment, there are
two entirely distinct classes of white fowls, of which one is
dominant and the other recessive to colour.
The Presence and Absence Hypothesis.—Whether the phenomenon
of dominance occur or not, the unit-characters exist
in pairs, of which the members are seemingly interchangeable.
In virtue of this behaviour the unit-characters forming such
a pair have been termed allelomorphic to one another, and
the question arises as to what is the nature of the relation
between two allelomorphs. The fact that such cases of heredity
as have been fully worked out can all be formulated in terms
of allelomorphic pairs is suggestive, and has led to what may
be called the “presence and absence” hypothesis. An allelomorphic
pair represents the only two possible states of any
given unit-character in its relation to the gamete, viz. its presence
or its absence. When the unit-character is present the
quality for which it stands is manifested in the zygote: when
it is absent some other quality previously concealed is able
to appear. When the unit-character for yellowness is present
in a pea the seeds are yellow, when it is absent the seeds are
green. The green character is underlying in all yellow seeds,
but can only appear in the absence of the unit-character for
yellowness, and greenness is allelomorphic to yellowness because
it is the expression of absence of yellowness.
Dihybridism.—The instances hitherto considered are all
simple cases in which the individuals crossed differ only in
one pair of unit-characters. Mendel himself worked out cases
in which the parents differed in more than one allelomorphic
pair, and he pointed out that the principles involved were
capable of indefinite extension. The inheritance of the various
allelomorphic pairs is to be regarded as entirely independent.
For example, when two individuals AA and aa are crossed
the composition of the F2 generation must be AA + 2Aa + aa.
If we suppose that the two parents differ also in the allelomorphic
pair B–b, the composition of the F2 generation for
this pair will be BB + 2Bb + bb. Hence of the zygotes which
are homozygous for AA one quarter will carry also BB, one
quarter bb, and one half Bb. And similarly for the zygotes
which carry Aa or aa. The various combinations possible
together with the relative frequencies of their occurrence may
be gathered from fig. 3. Of the 16 zygotes there are:—
9
containing
A and B
3
containing
B but not A
3
”
A but not B
1
”
neither A nor B
In a case of dihybridism the F1 zygote must be heterozygous for
the two allelomorphic pairs, i.e. must be of the constitution
AaBb. It is obvious that such a result may be produced in two
ways, either by the union of two gametes,
Ab and aB, or of two gametes AB and
ab. In the former case each parent
must be homozygous for one dominant
and one recessive character; in the
latter case one parent must be homozygous
for both the dominant and the
other for both recessive characters.
The results of a cross involving
dihybridism may be complicated in
several ways by the reaction upon one
another of the unit-characters belonging to the separate
allelomorphic pairs, and it will be convenient to consider
the various possibilities apart.
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. 3.
1. The simplest case is that in which the two allelomorphic
pairs affect entirely distinct characters. In the pea tallness
is dominant to dwarfness and yellow seeds are dominant to
green. When a yellow tall is crossed with a green dwarf the
F1 generation consists entirely of tall yellows. Precisely the
same result is obtained by crossing a tall green with a dwarf
yellow. In either case all the four characters involved are
visible in one or other of the parents. Of every 16 plants
produced by the tall yellow F1, 9 are tall yellows, 3 are tall
greens, 3 are dwarf yellows, and 1 is a dwarf green. If we
denote the tall and dwarf characters by A and a, and the yellow
and green seed characters by B and b respectively, then the
constitution of the F2 generation can be readily gathered from
fig. 3.
Fig. 4.
The four types of comb referred to in the text are shown here.
All the drawings were made from male birds. In the hens the
combs are smaller. All four types of comb are liable to a certain
amount of minor variation, and the walnut especially so. The
presence of minute bristles on its posterior portion, however,
serves at once to distinguish it from any other comb.
2. When the two allelomorphic pairs affect the same structure
we may get the phenomenon of novelties appearing in F1 and
F2. Certain breeds of fowls have a “rose” and others a “pea”
comb (fig. 4). On crossing the two a “walnut” comb
results, and the offspring of such walnuts bred together consist
of 9 walnuts, 3 roses, 3 peas, and 1 single comb in every
16 birds. This case may be brought into line with the scheme
in fig. 3 if we consider the allelomorphic pairs concerned to
