Generations of Painted Lady × Duke of Westminster cross.
Factors in Painted Lady × Duke of Westminster cross.
On the assumption that there exists a repulsion between the factors for erect standard and blue in a plant which is heterozygous for both, this peculiar case receives a simple explanation. The constitutions of the erect red and the hooded purple areEEbbandeeBBrespectively and that of the F1erect purple isEeBb. Now let us suppose that in such a zygote there exists a repulsionbetweenEandB, such that when the plant forms gametes these two factors will not go into the same gamete. On this view it can only form two kinds of gametes, viz.EbandeB, and these, of course, will be formed in equal numbers. Such a plant on self-fertilisation must give the zygotic seriesEEbb+ 2EeBb+eeBB,i.e.1 erect red, 2 erect purples, and 1 hooded purple. And because the erect reds and the hooded purples are respectively homozygous forEandB, they must thenceforward breed true. The erect purples, on the other hand, being always formed by the union of a gameteEbwith a gameteeB, are always heterozygous for both of these factors. They can, consequently, never breed true, but must always give erect reds, erect purples, and hooded purples in the ratio 1 : 2 : 1. The experimental facts are readily explained on the assumption of repulsion between the twofactorsBandEduring the formation of the gametes in a plant which is heterozygous for both.
Other similar cases of factorial repulsion have been demonstrated in the sweet pea, and two of these are also concerned with the two factors with which we have just been dealing. Two distinct varieties of pollen grains occur in this species, viz. the ordinary oblong form and a rather smaller rounded grain. The former is dominant to the latter.[7]When a cross is made between a purple with round pollen and a red with long pollen the F1plant is a long pollened purple. But the F2generation consists of purples with round pollen, purples with long pollen, and reds with long pollen in the ratio 1 : 2 : 1. No red with round pollen appears in F2owing to repulsion between the factors for purple (B) and for long pollen (L). Similarly plants produced by crossing a red hooded long with a red round having an erect standard give in F1long pollened reds with an erect standard, and these in F2produce the three types, round pollened erect, long pollened erect, and long pollened hooded, in the ratio 1 : 2 : 1. The repulsion here is between the long pollen factor (L) and the factor for the erect standard (E).
Yet another similar case is known in which we are concerned with quite different factors. In some sweet peas the axils whence the leaves and flower stalks spring from the main stem are of a deep red colour. In others they are green. The dark pigmented axil is dominant to the light one. Again, in some sweet peas the anthers are sterile, setting no pollen, and this condition is recessive to the ordinary fertile condition. When a sterile plant with a dark axil is crossed by a fertile plant with a light axil, the F1plants are all fertile with dark axils. But such plants in F2give fertiles with light axils, fertiles with dark axils, and steriles with dark axils in the ratio 1 : 2 : 1. No light axilled steriles appear from such a cross owing to the repulsion between the factor for dark axil (D) and that for the fertile anther (F).
These four cases have already been found in the sweet pea, and similar phenomena have been met with by Gregory in primulas. To certain seemingly analogous cases in animals where sex is concerned we shall refer later.
Now all of these four cases present a common feature which probably has not escaped the attention of the reader. In all of themthe original cross was such as to introduce one of the repelling factors with each of the two parents. If we denote our two factors byAandB, the crosses have always been of the natureAAbb×aaBB. Let us now consider what happens when both of thefactors, which in these cases repel one another, are introduced by one of the parents, and neither by the other parent. And in particular we will take the case in which we are concerned with purple and red flower colour, and with long and round pollen,i.e.with the factorsBandL. When a purple long (BBLL) is crossed with a red round (bbll) the F1(BbLl) is a purple with long pollen, identical in appearance with that produced by crossing the long pollened red with the round pollened purple. But the nature of the F2generation is in some respects very different. The ratio of purples to reds and of longs to rounds is in each case 3 : 1, as before. But instead of an association between the red and the long pollen characters the reverse is the case. The long pollen character is now associated with purple and the round pollen with red. The association, however, is not quite complete, and the examination of a large quantity of similarly bred material shows that the purple longs are about twelve times as numerous as the purple rounds, while the red rounds are rather more than three times as many as the red longs. Now this peculiar result could be brought about if the gametic series produced by the F1plant consisted of 7BL+ 1Bl+ 1bL+ 7blout of every 16 gametes. Fertilization between two such similar series of 16 gametes would result in 256 plants, of which 177 would be purple longs, 15 purple rounds, 15 red longs, and 49 red rounds—a proportion of the four different kinds very close tothat actually found by experiment. It will be noticed that in the whole family the purples are to the reds as 3 : 1, and the longs are also three times as numerous as the rounds. The peculiarity of the case lies in the distribution of these two characters with regard to one another. In some way or other the factors for blue and for long pollen become linked together in the cell divisions that give rise to the gametes, but the linking is not complete. This holds good for all the four cases in which repulsion between the factors occurs when one of the two factors is introduced by each of the parents.When both of the factors are brought into the cross by the same parent we get coupling between them instead of repulsion.The phenomena of repulsion and coupling between separate factors are intimately related, though hitherto we have not been able to suggest why this should be so.
Nor for the present can we suggest why certain factors should be linked together in the peculiar way that we have reason to suppose that they are during the process of the formation of the gametes. Nevertheless the phenomena are very definite, and it is not unlikely that a further study of them may throw important light on the architecture of the living cell.
APPENDIX TO CHAPTER IX
As it is possible that some readers may care, in spite of its complexity, to enter rather more fully into the peculiar phenomenonof the coupling of characters, I have brought together some further data in this Appendix. In the case we have already considered, where the factors for blue colour and long pollen are concerned, we have been led to suppose that the gametes produced by the heterozygous plant are of the nature 7BL: 1Bl: 1bL: 7bl. Such a series of ovules fertilised by a similar series of pollen grains will give a generation of the following composition:—
and as this theoretical result fits closely with the actual figures obtained by experiment we have reason for supposing that the heterozygous plant produces a series of gametes in which the factors are coupled in this way. The intensity of the coupling, however, varies in different cases. Where we are dealing with another, viz. fertility (F) and the dark axil (D), the experimental numbers accord with the view that the gametic series is here 15FD: 1Fd: 1fD: 15fd. The coupling is in this instance more intense. In the case of the erect standard (E) and blueness (B) the coupling is even more intense, and the experimental evidence available at present points to the gametic series here being 63Eb: 1EB: 1eB: 63eb. There is evidence also for supposing that the intensity of the coupling may vary in different families for the same pair of factors. The coupling between blue and long pollen is generally on the 7 : 1 : 1 : 7basis, but in some cases it may be on the 15 : 1 : 1 : 15 basis. But though the intensity of the coupling may vary it varies in an orderly way. IfAandBare the two factors concerned, the results obtained in F2are explicable on the assumption that the ratio of the four sorts of gametes produced is a term of the series—
In such a series the number of gametes containingAis equal to the number lackingA, and the same is true forB. Consequently the number of zygotes formed containingAis three times as great as the number of zygotes which do not containA; and similarly forB. The proportion of dominants to recessives in each case is 3 : 1. It is only in the distribution of the characters with relation to one another that these cases differ from a simple Mendelian case.
As the study of these series presents another feature of some interest, we may consider it in a little more detail. In the accompanying table are set out the results produced by these different series of gametes. The series marked by an asterisk have already been demonstrated experimentally. The first term in the series,in which all the four kinds of gametes are produced in equal numbers is, of course, that of a simple Mendelian case where no coupling occurs.
Now, as the table shows, it is possible to express the gametic series by a general formula (n+ 1)AB+Ab+aB+ (n- 1)ab, where 2nis the total number of the gametes in the series. A plant producing such a series of gametes gives rise to a family of zygotes in which 3n2- (2n- 1) show both of the dominant characters andn2- (2n- 1) show both of the recessive characters, while the number of the two classes which each show one of the two dominants is (2n- 1). When in such a series the coupling becomes closer the value ofnincreases, but in comparison withn2its value becomes less and less. The largernbecomes the more negligible is its value relatively ton2. If, therefore, the coupling were very close, the series 3n2- (2n- 1) : (2n- 1) : (2n- 1) :n2- (2n- 1) would approximate more and more to the series 3n2:n2,i.e.to a simple 3 : 1 ratio. Though the point is probably of more theoretical than practical interest, it is not impossible that some of the cases which have hitherto been regarded as following a simple 3 : 1 ratio will turn out on further analysis to belong to this more complicated scheme.
SEX
Fig. 17. Abraxas grossulariata varieties.Fig.17.Abraxas grossulariata, the common currant moth, and (on the right) its paler lacticolor variety.
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 sex. It was natural, therefore, that a comparison should have been early instituted 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 sexes 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 sexes is heterozygous for a factor which is lacking in the other, and that the presence or absence of this factor determines the sex 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 aslacticolor. 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 andrecessivesResults of crosses in Abraxas grossulariata.with regard to the sexes was peculiar. The original cross was between alacticolorfemale and a normal male. All the F1moths of both sexes were of the normalgrossulariatatype. The F1insects were then paired together and gave a generation consisting of 3 normals : 1lacticolor. But all thelacticolorwere females, and all the males were of the normal pattern. It was, however, found possible to obtain thelacticolor maleby mating alacticolorfemale with the F1male. The family resulting from this cross consisted of normal males and normal females,lacticolormales andlacticolorfemales, and thefour sorts were produced in approximately equal numbers. In such a family there was no special association of either of the two colour varieties with one sex rather than the other. But the reverse cross, F1female bylacticolormale, 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 normalgrossulariatawere males, while all thelacticolorwere females. Now this seemingly complex collection of facts is readily explained if we make the following three assumptions:—
(1) Thegrossulariatacharacter (G) is dominant to the lacticolor character (g). This is obviously justified by the experiments, for, leaving the sex distribution out of account, we get the expected 3 : 1 ratio from F1× F1, 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 constitution of a female is consequentlyFf, and of a maleff. This assumption is in harmony with the fact that the sexes are produced in approximately equal numbers.
(3) There exists repulsion between the factorsGandFin a zygote which is heterozygous for them both. Such zygotes (FfGg) must always be females, and on this assumption will produce gametesFgandfGin equal numbers.
Fig. 18. Scheme of inheritance for Abraxas grossulariata.Fig.18.Scheme of inheritance in the F1and F2generations resulting from the cross oflacticolorfemale withgrossulariatamale. The character of each individual is represented by the sex signs in brackets, the black beinggrossulariatain appearance and the light oneslacticolor.
Scheme of inheritance in the F1and F2generations resulting from the cross oflacticolorfemale withgrossulariatamale. The character of each individual is represented by the sex signs in brackets, the black beinggrossulariatain appearance and the light oneslacticolor.
We may now construct a scheme for comparison with that on page100to show how these assumptions explain the experimental results. The original parents werelacticolorfemale andgrossulariatamale, which on our assumptions must beFfggandffGGrespectively in constitution. Since the female is always heterozygous forF, her gametes must be of two kinds, viz.Fgandfg, while those of the puregrossulariatamale must be allfG. When an ovumFgis fertilised by a spermatozoonfG, the resulting zygote,FfGg, is heterozygous for bothFandG, and in appearance is a femalegrossulariata. The zygote resulting from the fertilisation of an ovumfgby a spermatozoonfGis heterozygous forG, but does not containF, and therefore is a malegrossulariata. Such a male being in constitutionffGgmust produce gametes of two kinds,fGandfg, in equal numbers. And since we are assuming repulsion betweenFandG, the F1female being in constitutionFfGg, must produce equal numbers of gametesFgandfG. For on our assumptionFandGcannot enter into the same gamete. The series of gametes produced by the F1moths, therefore, arefG,fgby the male andFg,fGby the female. The resulting F2generation consequently consists of the four classes of zygotesFfgg,FfGg,ffGg, andffGGin equal numbers. In other words, the sexes are produced in equal numbers, the proportion of normal grossulariata tolacticoloris 3 : 1, and all of thelacticolorare females; that is to say, the results worked out on our assumptions accord with those actually produced by experiment. We may now turn to the results which should be obtained by crossing the F1moths with thelacticolorvariety. And first we will take the crosslacticolorfemale × F1male. The gametes produced by the lacticolor female we have already seen to beFgandfg, while those produced by the F1male arefGandfg. The bringing together of these two series of gametes must result in equal numbers of the four kinds of zygotesFfGg,Ffgg,ffGg, andffgg,i.e.of femalegrossulariataandlacticolor, and of malegrossulariataandlacticolorin equal numbers. Here, again, the calculated results accord with those of experiment. Lastly, we may examine what should happen when the F1female is crossed with thelacticolormale. The F1female, owing to the repulsion betweenFandG, produces only the two kinds of ovaFgandfG, and produces them in equal numbers. Since thelacticolormale can contain neitherFnorG, all of its spermatozoa must befg. The results of such a cross, therefore, should be to produce equal numbers of the two kinds of zygoteFfggandffGg,i.e.oflacticolorfemales and ofgrossulariatamales. 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. Thelacticolorvariety in Great Britain is a southern form and is not known to occur in Scotland. Matings were made between wild Scotch females andlacticolormales. The families resulting from such matings were precisely the same as those fromlacticolormales and F1females, viz.grossulariatamales andlacticolorfemales only. We are, therefore, forced to regard the constitution of the wildgrossulariatafemale as identical with that of the F1female,i.e.as heterozygous for thegrossulariatafactor as well as for the factor for femaleness. Though from a region wherelacticoloris unknown, the "pure" wildgrossulariatafemale is nevertheless a permanent mongrel, but it can never reveal its true colours unless it is mated with a male which is either heterozygous forGor purelacticolor. And as all the wild northern males arepure for thegrossulariatacharacter this can never happen in a state of nature.
Fig. 19. Results of crossing Silky hen × Brown Leghorn cock.Fig. 19.Scheme illustrating the result of crossing a Silky hen with a Brown Leghorn cock. Black sex signs denote deeply pigmented birds, and light sex signs those without pigmentation. The light signs with a black dot in the centre denote birds with a small amount of pigment.
Scheme illustrating the result of crossing a Silky hen with a Brown Leghorn cock. Black sex signs denote deeply pigmented birds, and light sex 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.Lacticolorfemale ×grossulariatamale givesgrossulariataalone of both sexes. Butgrossulariatafemale ×lacticolormale gives onlygrossulariatamales andlacticolorfemales. 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 cock produced F1birds in which both sexes exhibited only traces of the pigment. On casual observation they might havepassed 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 F2male birds, however, showed the full deep pigmentation of the Silky.
Fig. 20. Results of crossing Brown Leghorn hen × Silky cock.Fig. 20.Scheme illustrating the result of crossing a Brown Leghorn hen with a Silky cock (cf. Fig. 19).
Scheme illustrating the result of crossing a Brown Leghorn hen with a Silky cock (cf. Fig. 19).
When, however, the cross was made the other way, viz. Brown Leghorn hen × Silky cock, the result was different. While the F1male birds were almost destitute of pigment as in the previous cross, the F1hens, on the other hand, were nearly as deeply pigmented as the pure Silky(Pl. V., 2). The male Silky transmitted the pigmentation, but only to his daughters. Such birds bred together gave an F2generation containing chicks with the full deep pigment, chicks without pigment, and chicks with various grades of pigmentation, all the different kinds in both sexes.
Fig. 21. Result of crossing F_1 birds with Brown Leghorn.Fig. 21.Scheme to illustrate the result of crossing F1birds (e.g.Brown Leghorn × Silky) with the pure Brown Leghorn.
Scheme to illustrate the result of crossing F1birds (e.g.Brown Leghorn × Silky) with the pure Brown Leghorn.
In analysing 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 F1birds and the pure Brown Leghorn. The cross between the F1hen and the Brown Leghorn cock 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 F1hen. But when the F1cock was mated to a Brown Leghorn hen, a definite proportion of the chicks, one in eight, was deeply pigmented, andthese deeply pigmented birds were always females(cf. Fig. 21). And in this respect all the F1males behaved alike, whether they were from the Silky hen or from the Silky cock. We have, therefore, the paradox that the F1hen, 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 F1cock can transmit this pigmented condition to a quarter of his female offspring though he himself is almost devoid of pigment.
Plate V.Plate V.1, 2, F1Cock and Hen, ex Brown Leghorn Hen × Silky Cock; 3, Silky Cock; 4, Hen ex Silky Hen × Brown Leghorn Cock.
1, 2, F1Cock and Hen, ex Brown Leghorn Hen × Silky Cock; 3, Silky Cock; 4, Hen ex Silky Hen × Brown Leghorn Cock.
Fig. 22. Scheme of inheritance for Silky hen × Brown Leghorn cock.Fig. 22.Scheme to illustrate the nature of the F1generation from the Silky hen and Brown Leghorn cock (cf. Fig. 23).
Scheme to illustrate the nature of the F1generation from the Silky hen and Brown Leghorn cock (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 assumptions (a) that there is repulsion betweenFandIin the female zygote (FfIi), and (b) that the male Brown Leghorn is homozygous 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 thegrossulariatafactor. 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 thepigmentation factor. In crossing a Silky hen with a Brown Leghorn cock we are mating two birds of the constitutionFfPPiiandffppII, and all the F1birds are consequently heterozygous for bothPandI. In such birds the pigment is almost but not completely suppressed, and as both sexes are of the same constitution with regard to these two factors they are both of similar appearance.
Fig. 23. Scheme of inheritance for Brown Leghorn hen × Silky cock.Fig. 23.Scheme to illustrate the nature of the F1generation from the Brown Leghorn hen and Silky cock (cf. Fig. 22).
Scheme to illustrate the nature of the F1generation from the Brown Leghorn hen and Silky cock (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 assumption is heterozygous for the inhibitor factor (I), and in constitution therefore isFfppIi. Owing to the repulsion betweenFandIthe gametes produced by such a bird areFpiandfpIin equal numbers. All the gametes produced by the Silky cock arefPi. Hence the constitution of the F1male birds produced by this cross isffPpIias before, but the female birds must be all of the constitutionFfPpii. The Silky cock 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 theinhibitory factor owing to the repulsion between these factors. The nature of the F2generation in each case is in harmony with the above scheme. As, however, it serves to illustrate 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 F1males and females when crossed with the Brown Leghorn. And, first, the cross of Brown Leghorn female by F1male. The Brown Leghorn hen is on our hypothesisFfppIi, and produces gametesFpiandfpI. The F1cock is on our hypothesisffPpIi, and produces in equal numbers the four kinds of gametesfPI,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 containsPin the absence ofI, and this is a female. The result, as we have already seen, is in accordance with the experimental facts.
Fig. 24. Scheme of inheritance for Brown Leghorn hen × F_1 cock.Fig.24.Diagram showing the nature of the offspring from a Brown Leghorn hen and an F1cock bred from Silky hen × Brown Leghorn cock, orvice versa.
Diagram showing the nature of the offspring from a Brown Leghorn hen and an F1cock bred from Silky hen × Brown Leghorn cock, orvice versa.
On the other hand, the Brown Leghorn cock is on our hypothesisffppII. All his gametes consequently contain the inhibitor factor, and when he is mated with an F1hen all the zygotes produced must containI. 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 homozygous or heterozygous forP.
Fig. 25. Scheme showing the heterozygous nature of the pure Brown Leghorn hen.Fig. 25.Scheme to illustrate the heterozygous nature of the pure Brown Leghorn hen. For explanation see text.
Scheme to illustrate the heterozygous nature of the pure Brown Leghorn hen. For explanation see text.
The interpretation of this case turns upon the constitution of the Brown Leghorn hen, upon her heterozygous condition with regard to the two factorsFandI, 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 constitutionppii. Such birds when crossed with the Silky give dark pigmented birds of both sexes in F1, and the F2generation consists of pigmented and unpigmented in the ratio 3 : 1. Now a cock 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 gametesFpiandfpI, the male birds produced by such a cross should be heterozygous forI,i.e.in constitutionffppIi, while the hen birds, though identical in appearance so far as absence of pigmentation goes, should not contain this factor but should be constitutionallyFfppii. Crossed with the pure Silky, the F1birds of opposite sexes should give an entirely different result. For while the hens should give only deeply pigmented birds of both sexes, the cocks 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 whichrepels 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 cock 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 analysis has shown, it is one of unequal transmission by the mother, the father's contribution to the offspring being identical for each sex. 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 homozygous 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,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.
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 isFfMM, and for a maleffMM. Both sexes are homozygous for the male element, and the difference between them is due to the presence or absence of the female elementF.
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 flyare inherited on the same lines as thegrossulariataandlacticolorpatterns 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.