Will it be a boy or a girl? ‘is a question that has been V V asked by so many anxious parents that there has never been a lack of prophets, or even of people who claim to control sex—some honest, some otherwise, but all equally wrong. But since ‘Boy or Girl ‘is the same as ‘Heads or Tails,’ half the answers are pretty well bound to be right, and the other half can always be explained away !
The real controllers of sex are, again, the chromosomes, which have so far been described as being ranged in one, two, or more pairs. This rule has one exception. There is one chromosome in the cells of a male which, though a sizeable, rod-like body itself, has a partner that looks, in comparison, no larger than a blotchy full-stop. The big one of this odd pair is called the X-chromosomc, the little one the Y-chromo-some. In the cells of the female, however, the X-chromosome is partnered by another X in every respect like itself. Now, disregarding all the other chromosome-pairs, see how the sex-chromosomes (as the X’s and Y’s together are called) behave in the breeding process.
The female cell is XX and can therefore only give rise to X ova. But the male cell is XY and can therefore give rise to two kinds of sperm—X and Y. If an X-sperm fertilises the X-ovum, the result will be XX, a female; and if a Y-spcrm does so, the result will be XY, a male. Thus chance, and chance alone decides, at the moment of conception, whether the child is to be a boy or a girl.
When this discovery was first made, biologists thought that it completely explained the approximate equality of the sexes
—since equal numbers of X and Y sperm should result in the birth of equal numbers of boys and girls.
The facts, though, are not quite so simple, since boy babies consistently outnumber girl babies by about 105 to 100. Moreover, counts of dead embryos show that in man—and probably in most mammals—nearly twice as many males as females are conceived, but that the male type of organism is inherently more likely to die. After birth, the male death rate of humans is consistently higher than the female, except during the short period of puberty, and over the age of 80 there are nearly twice as many women as men. The reason for the much higher male conception rate is still a mystery. But it is believed to be due to the Y-sperms being lighter and faster swimmers than the X’s, and being therefore more often successful in reaching and fertilising the ova.
In many species—cattle, for instance—the X-chromosome in male cells has no partner at all. A bull, that is to say, is XO, instead of XY, while the cow, of course, is XX. This fact (plus some more intricate evidence) makes us think that the little Y-chromosome, when it exists at all, is a mere ‘dummy,’ which plays no part at all in the life of the cell.
Essential maleness therefore consists in having one dose of the little bunch of chemicals called X, while essential female-ness consists in having two doses. There is an intriguing exception, not yet understood, to this general rule of sex-determination. All the birds, all the moths, and a few other species are exactly the opposite. The male is XX and the female XY.
CRISS-CROSS HEREDITY: THE CURIOUS FAMILY OF ‘BLEEDERS’ CRISS-CROSS heredity is the useful, old-fashioned term for something that puzzled everybody until the mechanism of sex-determination was discovered. An example explains it best, and the human disease called haemophilia is a good one, since most people have heard of hemophilics, who are popularly called ‘bleeders.’ A true bleeder is a man who may easily bleed to death from the slightest little cut, while anything like a large wound is almost certain to be fatal. One of the sons of King Alphonso of Spain died thus, having received some slight internal injury in a motor accident. The feature of this disease that was first noticed was that it only occurs in men. Next it was observed that all the children of such men were completely free from
it. Finally, it was found, the sons of their daughters often inherited it. Thus it went ‘criss-cross ‘—from bleeder father to non-bleeder daughter, and then across again to her bleeder son.
This was never explained until the function of the sex-chromosomes was grasped, when somebody pointed out that the X-chromosome was large, and therefore probably carried more genes than those concerned with sex-determination. If so, then any abnormality in one of them must inevitably be inherited in a criss-cross fashion; it must be ‘sex-linked.’ Bleeding is an example that has been fully investigated.
All the sons of this marriage, you can see, cannot help being normal, since they receive from their bleeder father only the little ‘dummy ‘Y-chromosome. All the daughters, though, cannot help receiving the bleeder X from him. but they—for a reason to be explained in a moment—do not show the deficiency. When one of them marries a normal man, however, she produces two kinds of ova. bleeder X and normal X, and four kinds of children are therefore possible.
Why a woman does not show the bleeder gene she carries, is best understood by tracing the disease backwards. The prolonged haemorrhage of haemophilia is due to the blood failing to clot when exposed to the air (normal blood takes about a minute). This failure, in its turn, is due to an almost complete lack of a substance called fibrinogen, which gives ‘body,’ as it were, to the blood. Fibrinogen is manufactured by that physiological maid-of-all-work, the liver; and a bleeder’s liver, though it is of normal size, seems to be in some ways like the liver of a child a good time before birth —before fibrinogen is needed or developed.
A bleeder’s liver is one that has never grown up ! Why? We have seen that each of us is, in a real sense, a double personality, since we have two chromosomes (and therefore two genes) for every job of work. But two are not always necessary, since one is often enough. A normal woman, for instance, clearly does not need both of this particular pair of genes, since one is all a normal man possesses. And a
woman with only one of them is no worse off than any normal man. But if a man gets that defective X, he is lost, inevitably a bleeder, since he has no normal X, as a female ‘carrier ‘has, to compensate for the deficiency. From the word ‘go ‘his liver completely lacks one of the chemicals essential for full development.
This instance, by the way, is a good example of the long chain of complex reactions between gene and character— from gene to pre-natal liver, from pre-natal to post-natal liver, from the last to fibrinogen, from fibrinogen to bleeding. And there must be many intermediate stages as well. Every gene, you may say, does its work in the same sort of indirect way. Tracking it down is like trying to trace in detail the activities of secretary, paper-makers, printers, carters, etc., when the manager of a firm gives a simple order for a new kind of notepaper !
SEX-LINKED CHARACTERS IN CATTLE
SEX-LINKED characters are not always confined to males. One of great commercial importance, recently discovered, appears only in females—milk-yield in cattle. A medium-yielding cow carries the gene with the ‘kick ‘in it in one of her X-chromosomes; a high-capacity cow has it in both. Taking the latter, she hands on a single dose of the character to every one of her children. The bulls, naturally, cannot show their dose at all, though they carry it (in their single X-chromosome). The cows will only be medium-yielders, unless their father is of the same grade as their brothers, so that they receive a dose from him as well as one from their mother.
To put it in another way, a cow inherits her milking qualities not only from her mother, but also, through her father, from her grandmother. Up till this discovery, a great many breeders had never bothered about the sire’s effect upon the milk-yield of his daughters—and consequently were always failing to breed champion milkers.
Since sex-determination in moths and birds is exactly the reverse of what it is in other species, sex-linkage is also topsyturvy—a fact to be remembered by anyone interested in poultry-breeding.