DISCOVERY 



233 



Recent Work on Heredity 



By J. S. Huxley, M.A. 



Fellow of S'eiv College, Oxford 



{Continued from the July No., p. 203) 

 (4) The Inheritanxe of Sex 



For long, the inheritance of sex was as much of a 

 mystery as sex itself. Even if we took maleness and 

 femaleness for granted, how was it that, in each genera- 

 tion, roughly equal numbers of males and females 

 were produced ? Though sex itself is still among the 

 more mysterious problems of biologj', sex-inheritance 

 has been shown to have a basis that is almost startling 

 in its simplicity. Putting the facts in terms of the 

 idea set forth in our last article, the idea that unit- 

 factors are arranged in a fixed order along the chromo- 

 somes, it appears that a sex-factor exists in a particular 

 chromosome, and that, while two of these sex-chromo- 

 somes are present in one sex, only one is found in the 

 other. This is the one exception to the rule that 

 chromosomes are always present in similar pairs. 



These exceptional chromosomes carrjang the sex- 

 factor are usually called the X-chromosomes. In 

 most animals, including man and other mammals, 

 the sex with the two X's is the female, while the male 

 has only one X. In the male, however, the ether 

 member of the pair is present, but is not similar to the 

 X in behaviour, and often not in shape or size. It is 

 called the Y-chromosome. It appears to be a chromo- 

 some that has been rendered inactive so far as taking 

 part in inheritance goes. This inactivity is often 

 followed, as in the case of other rudimentary' organs, 

 like the hind-limbs of whales or our own appendix, 

 by degeneration ; for the Y-chromosome is often 

 smaller than the X-chromosome, sometimes tiny, and 

 in a few cases altogether absent. 



Thus in mammals the difference between the sexes, 

 so far as chromosomes go, is that the female is XX, 

 the male XY. \\Tiat happens when the reproductive 

 cells are formed ? We have seen that, with ordinary 

 chromosomes, reduction occurs, the two members of 

 a pair being separated. In the female, since two 

 similar X's are present, one will be found in every 

 ovum. But in the male, X must separate from Y, 

 so that half the male cells wiU carrj' an X-chromosome, 

 and half a Y-chromosome. \\1ien an X-bearing sperm 

 fertihses an o\'um, the resultant individual will have 



two X's, and will therefore be a female ; while when 

 a Y-bearing sperm effects fcrtili^^ation, the resultant 

 will be XY, and therefore a male. 



A diagram will make this clear (see below). 



If we interpret this in terms of sex-factors, various 

 lines of evidence show that the real difference between 

 a male and a female is that the female possesses two 

 sex-units, two doses of sex-factor, while the male has 

 only one, the Y-chromosome being inactive. 



So far as it goes, this statement of the facts is 

 adequate enough, and explains how there comes to be, 

 on the average, an equality of the sexes in most animals. 



But a great many questions are left unanswered. 

 For instance, the males and females of the majority 

 of higher animals differ from each other not only in 

 the primary sex-characters, maleness and femaleness, 

 but also in a great many so-called secondary sexual 

 characters — for instance, the antlers of male deer ; 

 the bright colour of many cock birds ; the moustache 

 and beard of man ; the high voice and small hands and 

 feet of woman ; and the differences in instinct between 

 the sexes. .\re the factors for all these differences 

 simply part of the sex-factors, or are they quite separate, 

 but somehow controlled by the sex-factors ? There 



no doubt that the second alternative is the true one. 



To start with, wherever careful investigation has 

 been made, an individual of one sex can be shown to 

 contain the factors both for the secondary sex- 

 characters which it possesses, and also those for the 

 secondary sex-characters of the opposite sex, but in a 

 latent condition. For instance, if a hen Golden 

 Pheasant is crossed with an ordinary cock pheasant, 

 its male offspring will show, besides some male char- 

 acters derived from their father, others typical of the 

 male Golden Pheasant. These must have been 

 present, though invisible, in the mother. Again, when 

 the ovary of a bird is removed, at the next moult the 

 operated hen will appear with the plumage of a cock. 

 Most starthng of all, Steinach, in Germany, showed that, 

 if the reproductive organs are removed from a young 

 male rat, and ovaries from a young femalegrafted into 

 the abdomen in their place, the malegrows up into a 

 creature resembhng the female in every particular, 

 not merely in size and form, but also in instincts. 



We must therefore suppose that the sex-factor acts 

 like a switch ; one dose of it, in mammals, turning on 

 a set of conditions which permit of all the male char- 

 acters developing, two doses turning on the conditions 

 that allow femaleness and female characters to appear. 



Parent. 

 Female 



ReproducUve 

 Cells. 

 All contain X 



( 50 % contain X 

 V5o% contain Y 



Fertilisation. 



x+x=xx 



OITspring. 



: female (50%). 



X -f y = XY = male (50%) 



DIAGRAM OF SEX-DETBRMINATION BY MEANS OF THE CHROMOSOMES. 



