Mendelism Part 3

+-------+-------+-------+-------+ CB##### CB##### CB##### CB##### CB##### Cb##### cB##### cb##### ####### ####### ####### ####### ##BLACK ##BLACK ##BLACK ##BLACK +-------+-------+-------+-------+ Cb##### Cb..... Cb##### Cb..... CB##### Cb..... cB##### cb..... ####### ....... ####### ....... ##BLACK ...Blue ##BLACK ...Blue +-------+-------+-------+-------+ cB##### cB##### cB cB CB##### Cb##### cB cb ####### ####### ##BLACK ##BLACK +-------+-------+-------+-------+ cb##### cb..... cb cb CB##### Cb..... cB cb ####### ....... ##BLACK ...Blue +-------+-------+-------+-------+

FIG. 11.

Diagram to ill.u.s.trate the appearance of the reversionary blue pigeon in F_2 from the cross of black with white.

{68}

CHAPTER VII

DOMINANCE

[Ill.u.s.tration: FIG. 12.

Primula flowers to ill.u.s.trate the intermediate nature of the F_1 flower when _sinensis_ is crossed with _stellata_.]

Sinensis 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 h.o.m.ozygous form by mere inspection, however close. Breeding tests alone can decide which is the heterozygous and which the h.o.m.ozygous 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 ill.u.s.tration.

{70}

Blue Blue +-------------+--------+------------+ Black Blue Blue White +------+--+--+-----+ Black Black Blue Blue White White Black ------------- ------------- 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 h.o.m.ozygous 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 ill.u.s.trate 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 const.i.tution 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 const.i.tution 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: FIG. 15.

Fowls' feet. On the right a normal and on the left one with an extra toe.]

[Ill.u.s.tration: FIG. 16.

Scheme to ill.u.s.trate 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 s.e.xes, while the other, the Suffolk, is without horns in either s.e.x. 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 s.e.xes 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 s.e.x 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 dest.i.tute of horns.

[Ill.u.s.tration: PLATE III.

Sheep]

{79}

CHAPTER VIII

WILD FORMS AND DOMESTIC VARIETIES

In discussing the phenomena of reversion we have seen that in most cases such reversion occurs when the two varieties which are crossed each contain certain factors lacking in the other, of which the full complement is necessary for the production of the reversionary wild form. This at once suggests the idea that the various domestic forms of animals and plants have arisen by the omission from time to time of this factor or of that. In some cases we have clear evidence that this is the most natural interpretation of the relation between the cultivated and the wild forms.

Probably the species in which it is most evident is the sweet pea (_Lathyrus odoratus_). We have already seen reason to suppose that as regards certain structural features the Bush variety is a wild lacking the factor for the proc.u.mbent habit, that the Cupid is a wild without the factor for the long inter-node, and that the Bush Cupid is a wild minus both these factors. Nor is the evidence less clear for the many colour varieties. In ill.u.s.tration we may consider in more detail a case in which the cross between two whites resulted {80} in a complete reversion to the purple colour characteristic of the wild Sicilian form (Pl. IV.). In this particular instance subsequent breeding from the purples resulted in the production of six different colour forms in addition to whites. The proportion of the coloured forms to the whites was 9 : 7 (cf. p. 44), but it is with the relation of the six coloured forms that we are concerned here. Of these six forms three were purples and three were reds. The three purple forms were (1) the wild bicolor purple with blue wings known in cultivation as the Purple Invincible (Pl. IV., 4); (2) a deep purple with purple wings (Pl. IV., 5); and (3) a very dilute purple known as the Picotee (Pl. IV., 6). Corresponding to these three purple forms were three reds: (1) a bicolor red known as Painted Lady (Pl. IV., 7); (2) a deep red with red wings known as Miss Hunt (Pl. IV., 8); and (3) a very pale red which we have termed Tinged White[5] (Pl. IV., 9). In the F_2 generation the total number of purples bore to the total number of reds the ratio 3 : 1, and this ratio was maintained for each of the corresponding cla.s.ses.

Purple, therefore, is dominant to red, and each of the three cla.s.ses of red differs from its corresponding purple in not possessing the blue factor (B) which turns it into purple.

[Ill.u.s.tration: PLATE IV.

1, 2, Emily Henderson; 3, F_1 reversionary Purple; 4-10, Various F_2 forms: 4, Purple; 5, Deep Purple; 6, Picotee; 7, Painted Lady; 8, Miss Hunt; 9, Tinged White; 10, White.]

{81} Again, the proportion in which the three cla.s.ses of purples appeared was 9 bicolors, 3 deep purples, 4 picotees. We are, therefore, concerned here with the operation of two factors: (1) a light wing factor, which renders the bicolor dominant to the dark winged form; and (2) a factor for intense colour, which occurs in the bicolor and in the deep purple, but is lacking in the dilute picotee. And here it should be mentioned that these conclusions rest upon an exhaustive set of experiments involving the breeding of many thousands of plants. In this cross, therefore, we are concerned with the presence or absence of five factors, which we may denote as follows:--

A colour base, R.

A colour developer, C.

A purple factor, B.

A light wing factor, L.

A factor for intense colour, I.