CHAPTER XI

SEX (continued)

The cases which we have considered in the last chapter belong to a group in which the peculiarities of inheritance are most easily explained by supposing that the female is heterozygous for some factor that is not found in the male. Femaleness is an additional character superposed upon a basis of maleness, and as we imagine that there is a separate factor for each the full constitutional formula for a female is FfMM, and for a male ffMM. Both sexes are homozygous for the male element, and the difference between them is due to the presence or absence of the female element F.

There are, however, other cases for which the explanation will not suffice, but can be best interpreted on the view that the male is heterozygous for a factor which is not found in the female. Such a case is that recently described by Morgan in America for the pomace fly (Drosophila ampelophila). Normally this little insect has a red eye, but white eyed individuals are known to occur as rare sports. Red eye is dominant to white. In their relation to sex the eye colours of the pomace fly

are inherited on the same lines as the grossulariata and lacticolor patterns of the currant moth, but with one essential difference. The factor which repels the red-eye factor is in this case to be found in the male, and here consequently it is the male which must be regarded as heterozygous for a sex factor that is lacking in the female.

In order to bring these cases and others into line an interesting suggestion has recently been put forward by Bateson. On this suggestion each sex is heterozygous for its own sex factor only, and does not contain the factor proper to the opposite sex. The male is of the constitution, Mmff and the female Ffmm. Each sex produces two sorts of gametes, Mf and mf in the case of the male, and Fm, fm in that of the female. But on this view a further supposition is necessary. If each of the two kinds of spermatozoa were capable of fertilising each of the two kinds of ova, we should get individuals of the constitution MmFf and mmff, as well as the normal males and females, Mmff and Ffmm. As the facts of ordinary bisexual reproduction afford us no grounds for assuming the existence of these two classes of individuals, whatever they may be, we must suppose that fertilisation. is productive only between the spermatozoa carrying M and the ova without F, or between the spermatozoa

without M and the ova containing F. In other words we must on this view suppose that fertilisations between certain forms of gametes, even if they can occur, are incapable of giving rise to zygotes with the capacity for further development. If we admit this supposition, the scheme just given will cover such cases as those of the currant moth and the fowl, equally as well as that of the pomace fly. In the former there is repulsion between either the grossulariata factor and F, or else between the pigment inhibitor factor and F, while in the latter there is repulsion between the factor for red eye and M.

Scheme to illustrate the probable mode of inheritance of colour-blindness. The dark signs represent affected individuals. A black dot in the centre denotes an unaffected female who is capable of transmitting the condition to her sons.

Whatever the merits or demerits of such a scheme it certainly does offer an explanation of a peculiar form of sex limited inheritance in man. It has long been a matter of common knowledge that colour-blindness is much more common among men than among women, and also that unaffected women can transmit it to their sons. At first sight the case is not unlike that of the sheep, where the horned character is apparently dominant in the male but recessive in the female. The hypothesis that the colour-blind condition is due to the presence of an extra factor as compared with the normal, and that a single dose of it will produce

colour-blindness in the male but not in the female, will cover a good many of the observed facts (cf. Fig. 26). Moreover, it serves to explain the remarkable fact that all the sons of colour-blind women are also colour-blind. For a woman cannot be colour-blind unless she is homozygous for the colour-blind factor, in which case all her children must get a single dose of it even if she marries a normal male. And this is sufficient to produce colour-blindness in the male, though not in the female.

But there is one notable difference in this case as compared with that of the sheep. When crossed with pure hornless ewes the heterozygous horned ram transmits the horned character to half his male offspring (cf. p. [71]). But the heterozygous colour-blind man does not behave altogether like a sheep, for he apparently does not transmit the colour-blind condition to any of his male offspring. If, however, we suppose that the colour-blind factor is repelled by the factor for maleness, the amended scheme will cover the observed facts. For, denoting the colour-blind factor by X, the gametes produced by the colour-blind male are of two sorts only, viz. Mfx and mfX. If he marries a normal woman (Ffmmxx), the spermatozoa Mfx unite with ova fmx to give normal males, while the spermatozoa mfX unite with ova Fmx to give females which are heterozygous for the colour-blind factor. These daughters are themselves normal, but transmit the condition to about half their sons.

The attempt to discover a simple explanation of the nature of sex has led us to assume that certain combinations between gametes are incapable of giving rise to zygotes which can develop further. In the various cases hitherto considered there is no reason to suppose that anything of the sort occurs, or that the different gametes are otherwise than completely fertile one with another. One peculiar case, however, has been known for several years in which some of the gametes are apparently incapable of uniting to produce offspring. Yellow in the mouse is dominant to agouti, but hitherto a homozygous yellow has never been met with. The yellows from families where only yellows and agoutis occur produce, when bred together, yellows and agoutis in the ratio 2 : 1. If it were an ordinary Mendelian case the ratio should be 3 : 1, and one out of every three yellows so bred should be homozygous and give only yellows when crossed with agouti. But Cuénot and others have shown that all of the yellows are heterozygous, and when crossed with agoutis give both yellows and agoutis. We are led, therefore, to suppose that an ovum carrying the yellow factor is unproductive if fertilised by a spermatozoon which also bears this factor. In this way alone does it seem possible to explain the deficiency of yellows and the absence of homozygous ones in the families arising from the mating of yellows together. At present, however, it remains the only definite instance among animals in which we have

grounds for assuming that anything in the nature of unproductive fertilisation takes place.[[8]]

If we turn from animals to plants we find a more complicated state of affairs. Generally speaking, the higher plants are hermaphrodite, both ovules and pollen grains occurring on the same flower. Some plants, however like most animals, are of separate sexes, a single plant bearing only male or female flowers. In other plants the separate flowers are either male or female, though both are borne on the same individual. In others, again, the conditions are even more complex, for the same plant may bear flowers of three kinds, viz. male, female, and hermaphrodite. Or it may be that these three forms occur in the same species but in different individuals—female and hermaphrodites in one species; males, females, and hermaphrodites in another. One case, however, must be mentioned as it suggests a possibility which we have not hitherto encountered. In the common English bryony (Bryonia dioica) the sexes are separate, some plants having only male and others only female flowers. In another European species, B. alba, both male and female flowers occur on the same plant. Correns crossed these two species reciprocally, and also fertilised B. dioica by its own male with the following results:—

dioica ♀ × dioica ♂ gave ♀ ♀ and ♂ ♂
" × alba ♂ " ♀ ♀ only
alba ♀ × dioica ♂ " ♀ ♀ and ♂ ♂.

The point of chief interest lies in the striking difference shown by the reciprocal crosses between dioica and alba. Males appear when alba is used as the female parent but not when the female dioica is crossed by male alba. It is possible to suggest more than one scheme to cover these facts, but we may confine ourselves here to that which seems most in accord with the general trend of other cases. We will suppose that in dioica femaleness is dominant to maleness, and that the female is heterozygous for this additional factor. In this species, then, the female produces equal numbers of ovules with and without the female factor, while this factor is absent in all the pollen grains. Alba ♀ × dioica ♂ gives the same result as dioica ♀ × dioica ♂, and we must therefore suppose that alba produces male and female ovules in equal numbers. Alba ♂ x dioica ♀, however, gives nothing but females. Unless, therefore, we assume that there is selective fertilisation we must suppose that all the pollen grains of alba carry the female factor—in other words, that so far as the sex factors are concerned there is a difference between the ovules and pollen grains borne by the same plant. Unfortunately further investigation of this case is rendered impossible owing to the complete sterility of the F1 plants.

Single and double stocks raised from the same single parent.

That the possibility of a difference between the ovules and pollen grains of the same individual must be taken into account in future work there is evidence from quite a different source. The double stock is an old horticultural favourite, and for centuries it has been known that of itself it sets no seed, but must be raised from special strains of the single variety. "You must understand withall," wrote John Parkinson of his gilloflowers,[[9]] "that those plants that beare double flowers, doe beare

From all this it is clear enough that there is much to be done before the problem of sex is solved even so far as the biologist can ever expect to solve it. The possibilities are many, and many a fresh set of facts is needed before we can hope to decide among them. Yet the occasional glimpses of clear-cut and orderly phenomena, which Mendelian spectacles have already enabled us to catch, offer a fair hope that some day they may all be brought into focus, and assigned their proper places in a general scheme which shall embrace them all. Then, though not till then, will the problem of the nature of sex pass from the hands of the biologist into those of the physicist and the chemist.