Hormones and Heredity Part 5

Steinach considers that he has proved from results that s.e.x is not fixed or predetermined but dependent on the p.u.b.erty gland. By s.e.x here he obviously means the instincts and somatic characters, for s.e.x in the first instance, as we have already pointed out, means the difference between ovary and testis, between ova and spermatozoa. It is difficult to accept all Steinach's results without confirmation, especially those which show that the feminised male is more female than the normal female. Such a conclusion inevitably suggests that the investigator is proving too much.

The subject of the influence of hormones from the gonads is mentioned, but not fully discussed, in a volume by Dr. Jacques Loeb, ent.i.tles _The Organism as a Whole_. [Footnote: Putnam's Sons, 1916.] Loeb entirely omits the problem of the _origin_ of somatic s.e.x-characters, and fails to perceive that the fact that such characters are dependent to a marked degree on hormones derived from the gonads, together with their relation to definite habits and functions connected with the behaviour of the s.e.xes to each other, is proof are these characters are not gametogenic, but were originally due to external stimulation of particular parts of the soma.

CHAPTER IV

Origin Of Somatic s.e.x-Characters In Evolution

In his _Mendel's Principles of Heredity_, 1909, Bateson does not discuss the nature of somatic s.e.x-characters in general, but appears to regard them as essential s.e.x-features, as male or female respectively. As mentioned above, he argues from the fact that injury or disease of the ovaries may lead to the development of male characters in the female, that the female is heterozygous for s.e.x, and from the supposed fact that castration of the male leads merely to the non-appearance of male somatic characters, that the female s.e.x-factor is wanting in the male. He does not distinguish somatic s.e.x-characters from primary s.e.x-factors, and discusses certain cases of heredity limited by s.e.x as though they were examples of the same kind of phenomenon as somatic s.e.x-characters in general. One of these cases is the crossing by Professor T. B. Wood of a breed of sheep horned in both s.e.xes with another hornless in both s.e.xes. In the _F1_ generation the males were horned, the females hornless. Here, with regard to the horned character, both s.e.xes were of the same genetic composition, _i.e._ heterozygous, or if we represent the possession of horns by _H_, and their absence by _h_, both s.e.xes were _Hh_. Thus _Hh[male]_ was horned and _Hh[female]_ was hornless, or, as Bateson expresses it, the horned character was dominant in males, recessive in females. Bateson offers no explanation of this, but it obviously suggests that some trace of the original dimorphism of the sheep in this character was retained in both horned and hornless breeds. We may suppose that the factor for horns had disappeared entirely from the hornless sheep by a mutation, but in the horned breed another mutation had been a weakening of the influence of the s.e.xual hormones on the development of the character, which, as in all such cases, is really inherited in both s.e.xes. In the _F1_, when the horned character in the female is only inherited from one side, the hereditary tendency is not enough to overcome the influence of the absence of the testis hormone and presence of the ovarian hormone, and so the horns do not develop. The Mendelian merely sees a relation of the character to s.e.x, but overlooks entirely the question of the dimorphism in the original species from which the domesticated breeds are descended. Similarly, with regard to cattle where it has been found that hornlessness is dominant or nearly so in both s.e.xes, no reference is made to the opposite fact that wild cattle have horns in both s.e.xes and are not dimorphic in this character.

Bateson proceeds to consider colour-blindness as though its heredity were of similar kind. He refers to it as a male character latent in the female, remarks that we should expect that disease or removal of the ovaries might lead to the occasional appearance of colour-blindness in females. He also discusses the case of _Abraxas grossulariata_ and its variety _lacticolor_, and other cases of s.e.x-linked heredity, apparently with the idea that all such cases are similar to those of s.e.xual dimorphism. _A.

lacticolor_ occurs in nature only in the female s.e.x, and when bred with _grossulariata_ [male] produces [male]'s and [female]'s all _grossulariata_, these of course being heterozygous. When the _F1 grossulariata_ [male] was bred with the wild _lacticolor_ [female] it produced both forms in both s.e.xes, and thus _lacticolor_ [male] was obtained for the first time. When this _lacticolor_ [male] was bred with _F1 grossulariata_[female] it produced all the [male]'s _grossulariata_ and all the [female]'s _lacticolor_. Bateson's explanation is that the female, according to the Mendelian theory of s.e.x, is heterozygous in s.e.x, the male h.o.m.ozygous and recessive, and that _lacticolor_ is linked with the female s.e.x-character, _grossulariata_ being repelled by that character. Thus we have, the _lacticolor_ character being recessive,

lact. male, LL male male x F, gross. female, GL female male Gametes L male + L male x G male + L female _____________________|______________________ | | GL male male LL male female gross. male lact. female

It will be seen that although in the progeny of this mating all the _grossulariata_ were males and all the _lacticolor_ females, yet this case is by no means similar to that of s.e.xual dimorphism in which the characters are normally always confined to the same s.e.x. For the _lacticolor_ character in the parent was in the male, while in the offspring it was in the female. We cannot say here that in the theoretical factors which are supposed to represent what happens, the _lacticolor_ character is coupled with the female s.e.x-factor, for we find it with the male s.e.x-character in the _lacticolor_ [male]. It is so coupled only in the heterozygous _grossulariata_ [female], and at the same time the _grossulariata_ character is repelled.

According to Doncaster [Footnote: _Determination of s.e.x_, Camb. Univ.

Press, 1914.] s.e.x-limited, or as it is now proposed to call it s.e.x-linked, transmission in this case means that the female _grossulariata_ transmits the character to all her male offspring and to none of the female, while a heterozygous male _grossulariata_ mated with _lacticolor_ female transmits the character equally to both s.e.xes: that is to say, the heredity is completely s.e.x-limited in the female but not at all in the male. This is evidence that the female produces two kinds of eggs, one male producing and the other female producing.

With regard to the ordinary form of colour-blindness, Bateson's first explanation was that it was like the horns in the cross-bred sheep, dominant in males, recessive in females. About 4 per cent. of males in European countries are colour-blind, but less than 1/2 per cent. of females. Affected males may transmit the defect to their sons but not to their daughters: but daughters of affected persons transmit the defect frequently to their sons. Bateson gives [Footnote: _Mendel's Principles of Heredity_, 1909.] a scheme of the transmission, but corrects this in a note stating that colour-blindness does not descend from father to son, unless the defect was introduced by the normal sighted mother also, _i.e._ was carried by her as a recessive. The fact that unaffected males do not transmit the defect shows, according to Bateson, that it is due to the addition of a factor to the normal, not to omission of a factor.

According to later researches as quoted by Doncaster, colour-blindness is due to the loss of some factor which is present in the normal individual.

The normal male is heterozygous for this normal factor. If we denote the presence of the normal factor by _N_ and its absence or recessive by _n_, then the male is _Nn_, while the female is h.o.m.ozygous or _NN_. But in addition to this it is the male in this case which is heterozygous for s.e.x, and _n_ goes to the male-producing sperms, _N_ to the female-producing. Thus in the mating of normal man with normal woman the transmission is as follows:--

Nn (male) x NN (female) Gametes n (male) + N (female) x N + N

n (male) + N N (female) + N | | Nn (male) NN (female)

That is all offspring normal, but the males again heterozygous.

An affected male has the const.i.tution _nn_, and if he marries a normal woman the descent is as follows:--

nn (male) x NN (female) Gametes n (male) + n (female) x N + N

n (male) + N N (female) + N | | nN (male) nN (female)

When a normal male is mated with a heterozygous _nN_ female we get

nN (male) x nN (female) Gametes n (male) + N (female) x n + N ______________________|______________________ | | | | nn (male) nN (male) nN (female) NN (female)

that is, half the sons are normal and half colour-blind, while half the females are h.o.m.ozygous and normal, and the other half heterozygous and normal.

T. H. Morgan [Footnote: _A Critique of the Theory of Evolution._] has observed a number of cases of s.e.x-linked inheritance in the mutations which occurred in his cultures of _Drosophila_. The eye of the wild original fly is red, one of the mutants has a white eye, _i.e._ the red colour and its factor are absent. When a white-eyed male is mated to a red-eyed female all the offspring have red eyes. If these are bred _inter se_, there are, as in ordinary Mendelian cases, three red-eyed to one white-eyed in the _F2_ generation, but white eyes occur only in the males, in other wards half the males are white-eyed. On the other hand, when a white-eyed _female_ is mated to a red-eyed male all the daughters have red eyes, and all the sons white eyes. This has been termed crisscross inheritance. If these are bred together the result in _F2_ is equal numbers of red-eyed and white-eyed females, and equal numbers of red-eyed and white-eyed males. The ration of dominant to recessive is 2 to 2 instead of the usual Mendelian ration of 3 to 1.

According to Morgan the interpretation is as follows: In the nucleus of the female gametocytes there are two _X_ chromosomes related to s.e.x, in those of the male there is one _X_ chromosome and one _Y_ chromosome of slightly different shape. The factor for red eye occurs in the s.e.x-chromosomes, that is to say, according to this theory, the s.e.x-chromosome does not merely determine s.e.x but carries other factors as well, and this fact is the explanation of s.e.x-linked inheritance. The factor for red eye then is present in both _X_ chromosomes of the wild female, absent from both _X_ and _Y_ chromosomes of the white-eyed male.

The gametes of the female each carry one _X_ red chromosome, of those of the male half carry an _X_ white chromosome, and half the _Y_ white chromosome. The fertilised female ova therefore carry an _X_ red chromosome + an _X_ white chromosome, the male producing ova one _X_ red chromosome and one _Y_ white chromosome. They are all therefore red-eyed, but heterozygous--that is, the red eye is due to one red-eye factor, not two. When the _F1_ are bred together, half the female gametes carry one _X_ red chromosome, the other half one _X_ white chromosome; half the male gametes carry one _X_ red chromosome, the other half one _Y_ white chromosome. The fertilisations are therefore one _X_ red _X_ red, one _X_ red _X_ white, one _X_ red _Y_ white, and one _X_ white _Y_ white. These last are the white-eyed males. The two different crosses are represented diagrammatically below, the dark rod representing the _X_ red chromosome, the clear rod the _X_ white chromosome, and the bent clear rod the _Y_ white chromosome.

According to Morgan, the heredity of colour-blindness in man is to be explained exactly in the same way as that of white eye in _Drosophila_.

A colour-blind man married to a normal (h.o.m.ozygous) woman transmits the peculiarity to half his grandsons and to none of his grand-daughters.

Colour-blind women are rare, but in the few cases known where such women have married normal husbands the defect has appeared only in the sons, as in the second of the diagrams below.

Parents Red-eyed male White-eyed female XR XR x XW YW

F1 Red-eyed male Red-eyed female XR XW XR YW

F2 Red-eyed male Red-eyed male Red-eyed female White-eyed female XR XR XW XR XR YW XW YW h.o.m.ozygous. Heterozygous. Heterozygous. h.o.m.ozygous.

White-eyed male Red-eyed female XW XW x XR YW

F1 Red-eyed male White-eyed female XW XR XW YW

F2 White-eyed male Red-eyed male White-eyed female Red-eyed female XW XW XR XW XW YW XR YW h.o.m.ozygous. Heterozygous. h.o.m.ozygous. Heterozygous.

It must be explained that according to this theory the normal male is always heterozygous, because the _Y_ chromosome never carries any other factor except that for s.e.x; it is thus of no more importance than the absence of an _X_ chromosome which occurs in those cases where the male has one s.e.x-chromosome and the female two. According to the researches of von Winiwarter [Footnote: 'Spermatogenese humaine,' _Arch. de Biol._, xxvii., 1912.] on spermatogenesis in man, the latter is actually the case in the human species. This investigator found that there were 48 chromosomes in the female cell, 47 in the male; after the reduction divisions the unfertilised ova had 24 chromosomes, half the spermatids 24 and half 23, so that s.e.x is determined in man by the spermatozoon.

Morgan believes that the heredity of haemophilia (the const.i.tutional defect which prevents the spontaneous cessation of bleeding) follows the same scheme, and also at least some forms of stationary night-blindness-- that is, the inability to see in twilight.

We may mention a few other in animals, referring the reader for a fuller account to the works cited. One example in the barred character of the feathers in the breed of fowls called Plymouth Rock. In this the female is heterozygous for s.e.x as in _Abraxas grossulariata_, and the barred character is s.e.x-linked. When a barred hen is crossed with an unbarred c.o.c.k all the male offspring are barred, all the females plain. On the other hand, if a barred c.o.c.k is crossed with an unbarred hen, the barred character appears in all the offspring, both and females. The female thus transmits the character only to her sons. If we represent the barred character by _B_, and its absence by _b_, we can represent the heredity as follows:--

BARRED FEMALE WITH UNBARRED MALE

B female b male X b male b male

Bb male bb female

Barred male. Unbarred female.

Heterozygous. h.o.m.ozygous.

B male B male X b female b male

B male b female b male b male

Barred female. Barred male.

Heterozygous. Heterozygous.]

This case is thus exactly similar to that of _Abraxas grossulariata_ and _A. lacticolor_. The barred character like _grossulariata_ is dominant, the unbarred recessive, and to explain the results it is necessary to a.s.sume that the female is not only heterozygous for the barred character, but also for s.e.x, with the female s.e.x-factor dominant. The recessive character in this case is linked to the female s.e.x chromosome, or, as Bateson described it, the dominant character is repelled by the s.e.x-factor. We may make a diagram of the kind given by Morgan if we use a rod of different shape for the female-producing s.e.x-chromosome, and use the black rod for the dominant character:--

BARRED female x unbarred male BX uY uX uX | / | | / | BX uX uY uX BARRED male unbarred female Heterozygous h.o.m.ozygous

BARRED male x unbarred female BX BX uX uY | / | | / | BX uX BX uY BARRED male BARRED female Heterozygous Heterozygous

Another case is that of tortoise-sh.e.l.l, _i.e._ black and yellow cats. The tortoise-sh.e.l.l with very rare exceptions is female, the corresponding male being yellow, without any black colour. Doncaster found that a yellow male mated to a black female produced black male offspring and tortoise-sh.e.l.l females. When a black male is mated to a yellow female, the female kittens are tortoise-sh.e.l.l as before, but the males yellow. The Mendelian hypothesis which explains these results is that the male is always heterozygous, or has only one colour factor whether yellow or black, and transmits these colours only to his daughters, while the female has two colour factors, either _BB_, _YY_, or _BY_. Thus the crosses are:--