Page:EB1911 - Volume 18.djvu/136

 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:—

In a case of dihybridism the F1 zygote must be heterozygous for

the two allelomorphic pairs, i.e. must be of the constitution Aa Bb. 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.

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.



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