Mendelism Part 5

s.e.x

[Ill.u.s.tration: FIG. 17.

_Abraxas grossulariata_, the common currant moth, and (on the right) its paler lacticolor variety.]

In their simplest expression the phenomena exhibited by Mendelian characters are sharp and clean cut. Clean cut and sharp also are the phenomena of s.e.x. It was natural, therefore, that a comparison should have been early inst.i.tuted between these two sets of phenomena. As a general rule, the cross between a male and a female results in the production of the two s.e.xes in approximately equal numbers. The cross between a heterozygous dominant and a recessive also leads to equal numbers of recessives and of heterozygous dominants. Is it not, therefore, possible that one of the s.e.xes is heterozygous for a factor which is lacking in the other, and that the presence or absence of this factor determines the s.e.x of the zygote? The results of some recent experiments would appear to justify this interpretation, at any rate in particular cases. Of these, the simplest is that of the common currant moth (_Abraxas grossulariata_), of which there exists a pale variety (Fig. 17) known as _lacticolor_. The experiments of Doncaster and Raynor showed that the variety behaved as a simple recessive to the normal form. But the distribution of the dominants and {100} recessives [Ill.u.s.tration]with with regard to the s.e.xes was peculiar. The original cross was between a _lacticolor_ female and a normal male. All the F_1 moths of both s.e.xes were of the normal _grossulariata_ type. The F_1 insects were then paired together and gave a generation consisting of 3 normals : 1 _lacticolor_. But all the _lacticolor_ were females, and all the males were of the normal pattern. It was, however, found possible to obtain the _lacticolor male_ by mating a _lacticolor_ female with the F_1 male. The family resulting from this cross consisted of normal males and normal females, _lacticolor_ males and _lacticolor_ females, and the {101} four sorts were produced in approximately equal numbers. In such a family there was no special a.s.sociation of either of the two colour varieties with one s.e.x rather than the other. But the reverse cross, F_1 female by _lacticolor_ male, gave a very different result. As in the previous cross such families contained equal numbers of the normal form and of the recessive variety. But all of the normal _grossulariata_ were males, while all the _lacticolor_ were females. Now this seemingly complex collection of facts is readily explained if we make the following three a.s.sumptions:--

[Ill.u.s.tration]

(1) The _grossulariata_ character (G) is dominant to the lacticolor character (g). This is obviously justified by the experiments, for, leaving the s.e.x distribution out of account, we get the expected 3 : 1 ratio from F_1 F_1, and also the expected ratio of equality when the heterozygote is crossed with the recessive.

(2) The female is heterozygous for a dominant factor (F) which is lacking in the male. The const.i.tution of a female is consequently Ff, and of a male ff. This a.s.sumption is in harmony with the fact that the s.e.xes are produced in approximately equal numbers.

(3) There exists repulsion between the factors G and F in a zygote which is heterozygous for them both. Such zygotes (FfGg) must always be females, and on this a.s.sumption will produce gametes Fg and fG in equal numbers. {102}

[Ill.u.s.tration: FIG. 18.

Scheme of inheritance in the F_1 and F_2 generations resulting from the cross of _lacticolor_ female with _grossulariata_ male. The character of each individual is represented by the s.e.x signs in brackets, the black being _grossulariata_ in appearance and the light ones _lacticolor_.]

We may now construct a scheme for comparison with that on page 100 to show how these a.s.sumptions explain the experimental results. The original parents were _lacticolor_ female and _grossulariata_ male, which on our a.s.sumptions must be Ffgg and ffGG respectively in const.i.tution. Since the female is always heterozygous for F, her gametes must be of two kinds, viz.

Fg and fg, while those of the pure _grossulariata_ male must be all fG.

When an ovum Fg is fertilised by a spermatozoon fG, the resulting zygote, FfGg, is heterozygous for both F and G, and in appearance is a female _grossulariata_. The zygote resulting from the fertilisation of an ovum fg by a spermatozoon fG is heterozygous for G, but does not contain F, and therefore is a male _grossulariata_. Such a male being in const.i.tution {103} ffGg must produce gametes of two kinds, fG and fg, in equal numbers.

And since we are a.s.suming repulsion between F and G, the F_1 female being in const.i.tution FfGg, must produce equal numbers of gametes Fg and fG. For on our a.s.sumption F and G cannot enter into the same gamete. The series of gametes produced by the F_1 moths, therefore, are fG, fg by the male and Fg, fG by the female. The resulting F_2 generation consequently consists of the four cla.s.ses of zygotes Ffgg, FfGg, ffGg, and ffGG in equal numbers. In other words, the s.e.xes are produced in equal numbers, the proportion of normal grossulariata to _lacticolor_ is 3 : 1, and all of the _lacticolor_ are females; that is to say, the results worked out on our a.s.sumptions accord with those actually produced by experiment. We may now turn to the results which should be obtained by crossing the F_1 moths with the _lacticolor_ variety. And first we will take the cross _lacticolor_ female F_1 male. The gametes produced by the lacticolor female we have already seen to be Fg and fg, while those produced by the F_1 male are fG and fg.

The bringing together of these two series of gametes must result in equal numbers of the four kinds of zygotes FfGg, Ffgg, ffGg, and ffgg, _i.e._ of female _grossulariata_ and _lacticolor_, and of male _grossulariata_ and _lacticolor_ in equal numbers. Here, again, the calculated results accord with those of experiment. Lastly, we may examine what should happen when the F_1 female is crossed with the _lacticolor_ {104} male. The F_1 female, owing to the repulsion between F and G, produces only the two kinds of ova Fg and fG, and produces them in equal numbers. Since the _lacticolor_ male can contain neither F nor G, all of its spermatozoa must be fg. The results of such a cross, therefore, should be to produce equal numbers of the two kinds of zygote Ffgg and ffGg, _i.e._ of _lacticolor_ females and of _grossulariata_ males. And this, as we have already seen, is the actual result of such a cross.

Before leaving the currant moth we may allude to an interesting discovery which arose out of these experiments. The _lacticolor_ variety in Great Britain is a southern form and is not known to occur in Scotland. Matings were made between wild Scotch females and _lacticolor_ males. The families resulting from such matings were precisely the same as those from _lacticolor_ males and F_1 females, viz. _grossulariata_ males and _lacticolor_ females only. We are, therefore, forced to regard the const.i.tution of the wild _grossulariata_ female as identical with that of the F_1 female, _i.e._ as heterozygous for the _grossulariata_ factor as well as for the factor for femaleness. Though from a region where _lacticolor_ is unknown, the "pure" wild _grossulariata_ female is nevertheless a permanent mongrel, but it can never reveal its true colours unless it is mated with a male which is either heterozygous for G or pure _lacticolor_. And as all the wild northern males are {105} pure for the _grossulariata_ character this can never happen in a state of nature.

[Ill.u.s.tration: FIG. 19.

Scheme ill.u.s.trating the result of crossing a Silky hen with a Brown Leghorn c.o.c.k. Black s.e.x signs denote deeply pigmented birds, and light s.e.x signs those without pigmentation. The light signs with a black dot in the centre denote birds with a small amount of pigment.]

An essential feature of the case of the currant moth lies in the different results given by reciprocal crosses. _Lacticolor_ female _grossulariata_ male gives _grossulariata_ alone of both s.e.xes. But _grossulariata_ female _lacticolor_ male gives only _grossulariata_ males and _lacticolor_ females. Such a difference between reciprocal crosses has also been found in other animals, and the experimental results, though sometimes more complicated, are explicable on the same lines. An interesting case in which three factors are concerned has been recently worked out in poultry. The Silky breed of fowls is characterised among other peculiarities by a remarkable abundance of melanic pigment. The skin is dull black, while the comb and wattles are of a deep purple colour contrasting sharply with the white plumage (Pl. V., 3). Dissection shows that this black pigment is widely spread throughout the body, being especially marked in such membranes as the mesenteries, the periosteum, and the pia mater surrounding the brain. It also occurs in the connective tissues among the muscles. In the Brown Leghorn, on the other hand, this pigment is not found. Reciprocal crosses between these two breeds gave a remarkable difference in result. A cross between the Silky hen and the Brown Leghorn c.o.c.k produced F_1 birds in which both s.e.xes exhibited only traces of the pigment. On casual observation they might have {106} pa.s.sed for unpigmented birds, for with the exception of an occasional fleck of pigment their skin, comb and wattles were as clear as in the Brown Leghorn (Pl. V., 1 and 4). Dissection revealed the presence of a slight amount of internal pigment. Such birds bred together gave some offspring with the full pigmentation of the Silky, some without any pigment, and others showing different degrees of pigment.

None of the F_2 male birds, however, showed the full deep pigmentation of the Silky.

[Ill.u.s.tration: FIG. 20.

Scheme ill.u.s.trating the result of crossing a Brown Leghorn hen with a Silky c.o.c.k (cf. Fig. 19).]

When, however, the cross was made the other way, viz. Brown Leghorn hen Silky c.o.c.k, the result was different. While the F_1 male birds were almost dest.i.tute of pigment as in the previous cross, the F_1 hens, on the other hand, were nearly as deeply pigmented as the pure Silky {107} (Pl. V., 2).

The male Silky transmitted the pigmentation, but only to his daughters.

Such birds bred together gave an F_2 generation containing chicks with the full deep pigment, chicks without pigment, and chicks with various grades of pigmentation, all the different kinds in both s.e.xes.

[Ill.u.s.tration: FIG. 21.

Scheme to ill.u.s.trate the result of crossing F_1 birds (_e.g._ Brown Leghorn Silky) with the pure Brown Leghorn.]

In a.n.a.lysing this complicated case many other different crosses were made, but for the present it will be sufficient to mention but one of these, viz.

that between the F_1 birds and the pure Brown Leghorn. The cross between the F_1 hen and the Brown Leghorn c.o.c.k produced only birds with a slight amount of pigment and birds without pigment. And this was true for both the deeply pigmented and the slightly pigmented types of F_1 hen. But when the F_1 c.o.c.k was mated to a Brown Leghorn hen, a definite proportion of the chicks, one in eight, was deeply pigmented, and _these deeply pigmented birds were always females_ (cf. Fig. 21). And in this respect all the F_1 males behaved alike, whether they were from the Silky hen or from the Silky c.o.c.k. We have, therefore, the paradox that the F_1 hen, though herself deeply pigmented, cannot transmit this condition to any of her offspring when she is mated to the unpigmented Brown Leghorn, but that, when similarly mated, the F_1 c.o.c.k can transmit this pigmented condition to a quarter of his female offspring though he himself is almost devoid of pigment.

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

1, 2, F_1 c.o.c.k and Hen, ex Brown Leghorn Hen Silky c.o.c.k; 3, Silky c.o.c.k; 4, Hen ex Silky Hen Brown Leghorn c.o.c.k.]

{108}

[Ill.u.s.tration: FIG. 22.

Scheme to ill.u.s.trate the nature of the F_1 generation from the Silky hen and Brown Leghorn c.o.c.k (cf. Fig. 23).]

Now all these apparently complicated results, as well as many others to which we have not alluded, can be expressed by the following simple scheme.

There are three factors affecting pigment, viz. (1) a pigmentation factor (P); (2) a factor which inhibits the production of pigment (I); and (3) a factor for femaleness (F), for which the female birds are heterozygous, but which is not present in the males. Further, we make the a.s.sumptions (a) that there is repulsion between F and I in the female zygote (FfIi), and (b) that the male Brown Leghorn is h.o.m.ozygous for the inhibitor factor (I), but that the hen Brown Leghorn is always heterozygous for this factor just in the same way as the female of the currant moth is always heterozygous for the _grossulariata_ factor. We may now proceed to show how this explanation fits the experimental facts which we have given.

The Silky is pure for the pigmentation factor, but does not contain the inhibitor factor. The Brown Leghorn, on the other hand, contains the inhibitor factor, but not the {109} pigmentation factor. In crossing a Silky hen with a Brown Leghorn c.o.c.k we are mating two birds of the const.i.tution FfPPii and ffppII, and all the F_1 birds are consequently heterozygous for both P and I. In such birds the pigment is almost but not completely suppressed, and as both s.e.xes are of the same const.i.tution with regard to these two factors they are both of similar appearance.

[Ill.u.s.tration: FIG. 23.

Scheme to ill.u.s.trate the nature of the F_1 generation from the Brown Leghorn hen and Silky c.o.c.k (cf. Fig. 22).]

In the reciprocal cross, on the other hand, we are mating a Silky male (ffPPii) with a Brown Leghorn hen which on our a.s.sumption is heterozygous for the inhibitor factor (I), and in const.i.tution therefore is FfppIi.

Owing to the repulsion between F and I the gametes produced by such a bird are Fpi and fpI in equal numbers. All the gametes produced by the Silky c.o.c.k are fPi. Hence the const.i.tution of the F_1 male birds produced by this cross is ffPpIi as before, but the female birds must be all of the const.i.tution FfPpii. The Silky c.o.c.k transmits the fully pigmented condition to his daughters, because the gametes of the Brown Leghorn hen which contain the factor for femaleness do not contain the {110} inhibitory factor owing to the repulsion between these factors. The nature of the F_2 generation in each case is in harmony with the above scheme. As, however, it serves to ill.u.s.trate certain points in connection with intermediate forms we shall postpone further consideration of it till we discuss these matters, and for the present shall limit ourselves to the explanation of the different behaviour of the F_1 males and females when crossed with the Brown Leghorn. And, first, the cross of Brown Leghorn female by F_1 male.

The Brown Leghorn hen is on our hypothesis FfppIi, and produces gametes Fpi and fpI. The F_1 c.o.c.k is on our hypothesis ffPpIi, and produces in equal numbers the four kinds of gametes fPI, fPi, fpI, fpi. The result of the meeting of these two series of gametes is given in Fig. 24. Of the eight different kinds of zygote formed only one contains P in the absence of I, and this is a female. The result, as we have already seen, is in accordance with the experimental facts.

[Ill.u.s.tration: FIG. 24.

Diagram showing the nature of the offspring from a Brown Leghorn hen and an F_1 c.o.c.k bred from Silky hen Brown Leghorn c.o.c.k, or _vice versa_.]

On the other hand, the Brown Leghorn c.o.c.k is on our hypothesis ffppII. All his gametes consequently contain the inhibitor factor, and when he is mated with an F_1 {111} hen all the zygotes produced must contain I. None of his offspring, therefore, can be fully pigmented, for this condition only occurs in the absence of the inhibitor factor among zygotes which are either h.o.m.ozygous or heterozygous for P.

[Ill.u.s.tration: FIG. 25.

Scheme to ill.u.s.trate the heterozygous nature of the pure Brown Leghorn hen.

For explanation see text.]

The interpretation of this case turns upon the const.i.tution of the Brown Leghorn hen, upon her heterozygous condition with regard to the two factors F and I, and upon the repulsion that occurs between them when the gametes are formed. Through an independent set of experiments this view of the nature of the Brown Leghorn hen has been confirmed in an interesting way.

There are fowls which possess neither the factor for pigment nor the inhibitory factor, which are in const.i.tution ppii. Such birds when crossed with the Silky give dark pigmented birds of both s.e.xes in F_1, and the F_2 generation consists of pigmented and unpigmented in the ratio 3 : 1. Now a c.o.c.k of such a strain crossed with a Brown Leghorn hen should give only completely unpigmented birds. But if, as we have supposed, the Brown Leghorn hen is producing gametes Fpi and fpI, the male birds produced by such a cross should be heterozygous for I, {112} _i.e._ in const.i.tution ffppIi, while the hen birds, though identical in appearance so far as absence of pigmentation goes, should not contain this factor but should be const.i.tutionally Ffppii. Crossed with the pure Silky, the F_1 birds of opposite s.e.xes should give an entirely different result. For while the hens should give only deeply pigmented birds of both s.e.xes, the c.o.c.ks should give equal numbers of deeply pigmented and slightly pigmented birds (cf.

Fig. 25). These were the results which the experiment actually gave, thus affording strong confirmation of the view which we have been led to take of the Brown Leghorn hen. Essentially the poultry case is that of the currant moth. It differs in that the factor which {113} repels femaleness produces no visible effect, and its presence or absence can only be determined by the introduction of a third factor, that for pigmentation.

This conception of the nature of the Brown Leghorn hen leads to a curious paradox. We have stated that the Silky c.o.c.k transmits the pigmented condition, but transmits it to his daughters only. Apparently the case is one of unequal transmission by the father. Actually, as our a.n.a.lysis has shown, it is one of unequal transmission by the mother, the father's contribution to the offspring being identical for each s.e.x. The mother transmits to the daughters her dominant quality of femaleness, but to balance this, as it were, she transmits to her sons another quality which her daughters do not receive. It is a matter of common experience among human families that in respect to particular qualities the sons tend to resemble their mothers more than the daughters do, and it is not improbable that such observations have a real foundation for which the clue may be provided by the Brown Leghorn hen.

Nor is this the only reflection that the Brown Leghorn suggests. Owing to the repulsion between the factors for femaleness and for pigment inhibition, it is impossible by any form of mating to make a hen which is h.o.m.ozygous for the inhibitor factor. She has bartered away for femaleness the possibility of ever receiving a double dose of this factor. We know that in some cases, as, for example, {114} that of the blue Andalusian fowl, the qualities of the individual are markedly different according as to whether he or she has received a single or a double dose of a given factor. It is not inconceivable that some of the qualities in which a man differs from a woman are founded upon a distinction of this nature. Certain qualities of intellect, for example, may depend upon the existence in the individual of a double dose of some factor which is repelled by femaleness.

If this is so, and if woman is bent upon achieving the results which such qualities of intellect imply, it is not education or training that will help her. Her problem is to get the factor on which the quality depends into an ovum that carries also the factor for femaleness.

{115}

CHAPTER XI