The Natural Superiority of Women (24 page)

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Authors: Ashley Montagu

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the differences in sex characteristics; it
is
to say that the chromosomes are decisive in determining whether an organism shall develop as a male or a female. The sex chromosomes regulate the transformation of the zygote, or diploid cell, resulting from the union of the male and female gametes, or haploid cells, each carrying half the number of chromosomes.
Fertilization,
the word usually employed to describe this process, is not the best term, for the essence of the process is the fusion of two cells which will develop into an embryo, not the supremacy of one cell over another. During the first few weeks of development the embryo remains sexually undifferentiated, though oriented toward femaleness. Up to the end of the sixth week of embryonic development the appearance of the external genitalia is identical in the two sexes. If the embryo is a genetic male, masculinizing organizing substances will enlarge the phallus, extend the urethra along its length, and close the skin over the urogenital sinus to form the scrotum for the testes, which will later descend into it. In the absence of the masculinizing hormone testosterone (which is normally derived from the gonad, the sexually indifferent organ that may develop either as an ovary or testes), the infant will develop as a female, even though a female organizing substance does not exist. This, as the distinguished experimental endocrinologist Dr. Alfred Hoet and others have suggested, indicates that the basic surviving human form is female and that masculinity is something "additional."

1

Under normal conditions the sex rudiments are differentially affected toward maleness or femaleness depending upon whether the chromosomal constitution (the genotype) is XY or XX. The genotype of chromosomal constitution therefore is decisive in initiating the direction of sexual development; thereafter it is a matter largely under the influence of the developing hormones secreted by the endocrine glands. The development of all bodily structures and their functions, in relation to the environment in which they develop, is set by the sex chromosomes at the time the sex rudiments and the gonads are sexually differentiated. As Professor N. J. Berrill has written,
In any case, the status of the female is never in doubt. Whoever produces eggs is essential to the future, for eggs are reproductive cells, whatever else they may be. Sperm are not so in the primary sense of the word. They serve two decidedly

 

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secondary endsthey serve to stimulate the otherwise comatose eggs to start developing, like the kiss that awakened the Sleeping Beauty, and they serve to introduce considerable variability derived from the male parent.

2

Eggs, of course, also contribute their variability to the offspring, but eggs alone have the capacity, under certain conditions, to develop readily into grown organisms, whereas sperms lack such a capacity altogether. In all sexual species the mature organism is the developed egg, with the extra touches added, usually but not always, only when a sperm is involved.
What is the difference between an XX and an XY cell? When one looks at a body cell containing a full complement of forty-six chromosomes there is no difficulty in recognizing the XX sex chromosomes because they belong with the group of quite large chromosomes. But if one examines a body cell with the XY complement of the male, say at a magnification of two thousand diameters, it will be seen that the Y chromosome is the smallest of the forty-six chromosomes. It is in that difference, and what it signifies, that there lies part of the answer to the question, How do the sexes get that way?
The chromosomes which are neither X nor Y (twenty-two in the haploid and forty-four in the diploid state) are called
autosomes .
There are twenty-two pairs of them in the body cells, but only twenty-two
single
ones in the sex cells. Each of the autosomes contains factors that tend toward the production of femaleness. Each of the X chromosome contains genes that tend toward the production of femaleness. The Y chromosome carries factors that are male-determining. (Among the genes contained in the Y chromosome is one responsible for the secretion of the masculinizing hormone, testosterone, from the gonads.) Hence, when a Y-carrying sperm fuses with an ovum, the XY chromosomes, in the presence of the twenty-two pairs of autosomes carrying genes directed toward femaleness, are insufficiently powerful to reduce the influence of those genes, and the result is the development of a male. On the other hand, the combination of two X chromosomes, one from the mother and the other from the father, is sufficient to overcome the influence of any possibly male factors in the autosomes, and the result is a female. The X chromosomes together have quite a pull to them; and the explanation of the biological superiority of the female lies in the

 

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female having two X chromosomes while the male has only one.

3
It is largely to the original X chromosome deficiency of the male that almost all the troubles to which he falls heir may be traced, and to the presence of two well-appointed, well-furnished X chromosomes that the female owes her biological superiority. As a consequence of the larger size of the X chromosome, the female's cells are about 4 percent greater in chromosome volume than those of the male. As Drs. J. H. Tjio and T. T. Puck, who originally determined the difference in sex-chromosome size in 1958, have remarked, "The female has a substantially richer genetic capacity than the male."
4

The vital importance of the X chromosome as compared with the Y chromosome is unequivocally clear because no cell can survive long unless it contains an X chromosome. No matter how many Y chromosomes a cell may contain, if it does not also contain an X chromosome it dies. Males, therefore, survive only by grace of their having been endowed by their mothers with an X chromosome. In birds and some insects two X chromosomes produce a male and an XY combination produces a female, but otherwise the conditions are precisely the same as in humans, except that the autosomes contain the sex genes that are strongly organized toward femaleness, whereas the X chromosomes are strongly, and in double dose, more powerfully organized toward maleness. That it is the X chromosome that counts is borne out of by the incidence of embryo deaths, which is much greater among the female birds than among the males.
What the origin of the X and Y chromosomes may have been no one knows. It may be that the Y chromosome represents a remnant of an X chromosome. While the Y chromosome is the masculinizing agent, feminization will occur only when it is absent, while survival is impossible in the presence of a Y chromosome alone. On the other hand, development will occur in the presence of only one X chromosome, giving rise to a female with a condition known as Turner's syndrome, usually sterile and exhibiting a sort of webbing of the neck.
Thus far some twenty conditions have been traced to genes which sometimes occur only by fathers to their sons. Among these are barklike skin
(ichthyosis hystrix gravior),
dense hairy growth on the ears
(auricular hypertrichosis),
nonpainful hard lesions of the hands and feet
(keratoma dissipatum),
and a form

 

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of webbing of the toes in which there is fusion of the skin between the second and third toes.
It is probable that the biological disadvantages accruing to the male are not so much due to what is in the Y chromosome as to what is not in it. This is well exemplified by the manner in which the male inherits a serious disorder such as hemophilia (spontaneous or traumatic subcutaneous and intramuscular bleeding). This is due to a mutant gene carried on the X chromosome. A mutant gene is one in which a physicochemical change of a heritable kind occurs. It has been calculated that the normal gene for blood clotting mutates to the defective hemophilia gene in one out of every hundred thousand persons of European origin in each generation. Since most hemophiliacs die before they can leave any offspring, the number of such unfortunate persons alive at any time is relatively small. Hemophilia is inherited as a single, sex-linked recessive gene, that is, a gene that is linked to the X chromosome and that will not express itself in the presence of a normal gene on the opposite X chromosome. When, then, an X chromosome that carries the hemophilia gene is transmitted to a female, it is highly improbable that it will encounter another X chromosome carrying a similar defective gene; it is for this reason that hemophilia is of very great rarity in females. Since the survival rate of hemophiliacs to reproductive age is very low, it is obvious why females are the most unusual transmitters of the hemophilia gene, and it should also be clear why females practically never exhibit the condition. The males are affected because they lack the blood clotting factor on their Y chromosome which, were it present, would compensate for the deficiency in the X chromosome inherited from the mother. Women exhibit the condition only if they inherit one hemophilia gene from their mother and another from their father, but this is extremely unlikely to occur. If she derived a single defective X chromosome from either her father or her mother, the female would not suffer from hemophilia because her normal X chromosome would either compensate for, inhibit, or suppress the action of the hemophilia gene on the other X chromosome; if she then married a normal man and bore a number of children she would pass on the hemophilia-bearing chromosome to about half her sons and half her daughters. The girls who inherit the defective gene will show no ill effects, but the males who have received the gene may show the effects even

 

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