Page:EB1911 - Volume 18.djvu/137

 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.

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.

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