In medieval times, an obliging wife whose husband desired a son might accompany him to the village alchemist when the moon was full and bravely swallow a vile mixture of wine and lion`s blood, after which the couple would make love. It probably worked about half the time.
Aristotle believed that the human embryo was formed by a blend of semen and menstrual blood (a plausible belief at the time because menstruation ceases during pregnancy). Therefore, the only way to beget a baby boy was by a greater display of sexual ”vigor” by the would-be dad. Men were spurred to heroic heights of copulation, especially when a north wind was blowing. Aristotle`s theory survived in various guises well into the 1900s.
Males traditionally have been the preferred outcome of sex determination schemes, a lesson painfully learned by such queens as Anne Boleyn, beheaded when she failed to deliver. For centuries, male babies have been linked to the region of the right-that side of the conjugal bed, the female right ovary, the male right testicle. Females have been considered products of the left.
Ever-helpful Hippocrates advised fathers who wanted a boy to firmly elevate their starboard testicle, lashing it tightly with a cord before having sex. Even today in Hungary and Bavaria a wife may pinch her husband`s right testicle during intercourse, while Italian husbands are said to bite their lover`s right ear.
Again, it probably works about half the time.
Now David Page, a young molecular biologist at the Whitehead Institute for Biomedical Research in Cambridge, Mass., has found something that works every time.
Page, 31, says that the deep mystery of human gender may boil down to a tiny gene-a small molecule of DNA (deoxyribonucleic acid)-that all normal males carry on their Y, or sex, chromosome.
The elusive sex gene, ardently pursued by science for the last decade, triggers the complex cascade of chemical events that differentiate men and women.
A developing embryo has all the genetic instructions it needs to become a human being but is sexless until about the fifth week after conception. In essence, if the Y chromosome is present, the gene switches on and the result will be a baby boy. If the Y is not present to send the proper protein signals, it`s a girl.
”Before then, the embryo is indifferent; it shows no signs of gender,”
Page says. ”Once the gene blazes into action, it tells the fetal gonads to establish the hormonal environment that will make testicles. Otherwise, the gonads will develop into ovaries.”
Moreover, Page narrowed this down even finer. He has proved that only the minuscule part of the Y chromosome bearing the sex gene must be present to result in a male. Lion`s blood, pinches, vigor, full moon-how romantic they seem when compared to a relatively mindless molecular switch.
”But once made, the decision is final,” says Page. ”The die is cast.” A 1986 winner of a MacArthur Foundation prize, Page played hide and seek with the sex gene for six years. He calls it, unromantically, the TDF gene
(for testes determining factor), and its discovery hinged the powerful new techniques of genetic engineering.
Every cell in the human body has 23 pairs of chromosomes in its nucleus, or a total of 46. Because they`re intertwined like squirming spaghetti it wasn`t until 1956 that we could finally separate and count them. Until then, 48 was the best guess. But 46 it has turned out to be. The chromosomes have our genes, and the genes have our heredity.
Life starts out with sex cells-the sperm and egg-that have only 23 chromosomes each. These hold the contribution of both parents, and their parents, and of parents long dead and forgotten.
Among these 23, a female egg has one special chromosome responsible for all sex-linked traits. It is called X. Eggs can only be X.
But sperm cells are of two kinds-X and Y-the latter a stunted little chromsome whose runtlike appearance belies its remarkable power.
Only one hardy sperm cell will win the marathon race to the egg. If an X- sperm makes it, the fertilized egg will then have 46 chromosomes, including XX. If a Y-sperm wins, the embryo will be XY.
X+Y
a boy.
X+X
a girl.
Page`s discovery, however, hinged on rare and mysterious cases that break the rules. He studied people whose sexual identities appeared to conflict with their chromosomal makeup.
Some males (about 1 in every 20,000) possess two female X chromosomes.
And some females (about 1 in 3,000) carry both a female X and a male Y.
”Such XX males appear completely normal,” Page says. ”Their unique chromosomal makeup usually isn`t noticed until they marry and find they are unable to have children.
”The XY females, who have what we call Turner`s syndrome, may have short stature. They don`t menstruate and are sterile. But as with XX men, they otherwise are normal and tend to live long lives.
”From my point of view, however, they were fascinating and a total mystery.”
Page is one of a handful of gene splicers at the Whitehead, a supercharged biology think tank affiliated with the Massachusetts Institute of Technology. David Baltimore, director of the Whitehead, was a young wonder himself when, at 37, he won the 1975 Nobel Prize (with Americans Renato Dulbecco and Howard Temin) for discoveries concerning the interaction of tumor viruses and the genetic material of cells.
Baltimore calls Page`s gene ”clearly a landmark observation. It brings us knowledge for the first time of a gene that is a key to determining sex. It doesn`t tell us anything about the differences between males and females, but it tells us how it is first laid down and may lead us to a tremendous amount of knowledge about the differences.”
Page`s work links directly to 1910 and experimental geneticist Thomas Hunt Morgan, who thought he could learn something about sex determination by studying insects.
The work in Morgan`s lab at Columbia University later would win him the Nobel Prize. He and his students studied inheritance using what became the workhorse of genetics-Drosophila melanogaster, the so-called ”lover of the dew,” which in truth would sell its soul for rotting yeast-the fruit fly.
These tiny benign beings, about an eighth of an inch long, usually are found hovering around fruit bowls. They breed fast, don`t eat much and can be raised by the thousands in glass bottles with cork stoppers. Drosophila live and die in two weeks, so genetic traits may be observed easily among generations. They have scores of physical characteristics that can be mutated by X-rays and the changes studied, yet they are constructed from a relatively simple set of only four pairs of chromosomes.
By euthanizing the bugs with ether, grinding up their corpses and dissolving everything else, scientists retrieve their chromosomes. Since Morgan, untold millions of fruit flies have given their genes to science, and the similarity between theirs and ours has proven humbling.
”Morgan discovered that the chromosomal makeup in male and female fruit flies was different,” Page says. ”Sex was determined by the number of X chromsomes. Females had two XXs, males had only one.
”Later, in 1923, a cell biologist named Theodosius Painter discovered that humans had X and Y chromosomes. But for the next several decades, science thought we were like flies and that only the number of X chromosomes mattered. The Y chromosome was deemed irrelevant until 1959.”
The late 1950s proved a golden age for chromosome researchers because science had learned to distinguish the chromosomes after isolating them from bone marrow or blood cells. Chromosomal defects could then be isolated. The first to be understood was that victims of the Down syndrome had three copies of chromsome 21, instead of the normal two.
”Then a few males were found who had the Klinefelter syndrome-three sex chromosomes, or XXY,” Page says. ”Here was something that contradicted the the fruit fly studies: a male with two female X chromosomes and a Y. Gradually, it became understood that in humans the Y chromosome, not the X, confers maleness.”
Page, who would have been about 3 years old when the Y rose to prominence, calls his lab the XY Corral. He originally went to Harvard medical school to become a cardiologist and earned his M.D., but genetics research kept pulling at him.
In 1981 he was working with MIT scientists on piecing together the first rough map of the known genes that humans have (the completed project was announced last November). Maps are constructed from DNA probes that chemically seek out genes and hook onto them like magnets.
Early in that research, Page happened upon a previously unknown probe that matched up with genetic material found on both the X and Y chromosomes. He had found a special tool.
”I was so interested in pursuing the sex gene that I was no longer a very good soldier on the march to the gene map,” he says.
He reasoned that in males with two X chromosomes, a piece of the Y has to be hidden someplace; otherwise testes would not develop.
Accordingly, XY females must be missing some crucial part of the Y, because they still have ovaries.
”Therefore, in both sexes, the stretch of genetic material on the Y chromosome that defines sex, by its presence or absence, has to contain the TDF gene.”
Aided by two colleagues, Albert de la Chapelle of the University of Helsinki and Jean Weissenbach of the Pasteur Institute in Paris, Page began seeking pieces of the Y chromosome in XX men.
When he unleashed Y chromosome probes on the XXs, sure enough the probes bound to the hidden Y-DNA. By 1987, Page knew that all XX men carried a piece of the Y.
The mutation probably occurred during the creation of sperm, he speculates: The father`s sperm cell had carried an X chromosome and was destined to make a girl; but a bit of the Y had become attached to the X, and even though the sperm had paired with the mother`s X, the TDF gene was so powerful that it still ordered up a male baby.
The researchers next tried the same technique on XY females, screening their DNA to see which piece of the Y they were missing.
Eventually, after much experimentation, it was discovered that all the XY females were missing the same piece of the Y that the XX men had.
”We started out with a handful of patients and a few probes,” Page says. ”As the process improved, our precision improved, and all told we examined the DNA from about 90 sex-reversed patients.”
Two in particular led Page to the gene.
One XX male contained a mere 0.5 per cent of the Y chromosome. All the other males contained that stretch of Y-DNA, too.
Then a 12-year-old XY girl was found to contain 99.8 per cent of the Y. That missing stretch that kept her female matched up perfectly with the Y-DNA of the XX males, thus building what Page calls ”a strong circumstantial case” for the TDF gene.
”In the final analysis, the gene has turned out to occupy only 1/500th of the Y chromosome. That`s not very much. But it`s enough.”
As the researchers checked and rechecked their work, they performed what Page calls ”a Noah`s ark experiment,” examining the sex chromosomes of a host of nature`s creatures.
When they recently announced the gene in a historic and elegant paper published in the journal Cell, the scientists quoted the seventh chapter of Genesis, verses 8-9. Into their ark of genes ”went one pair, male and female, of (many) beasts, clean and unclean, of birds and (many things) that crawl on the ground, two by two.”
Humans were found to share the TDF gene with male chimpanzees, rhesus monkeys, owl monkeys, rodents, rabbits, cats, dogs, goats, horses, cattle and probably all other mammals.
Page`s gene has caused a sensation in scientific circles, and not only because it concerns sex. The TDF gene is the first so-called master regulatory gene to be pinpointed in the human body. It may help biology answer the question that has been gnawing at science since Aristotle: How does a complex organism arise from a single microscopic fertilized egg?
”We`ve found such master genes in viruses and yeast,” Page says. ”But unless I`m overlooking something, this is the first one to be found in people.”
The TDF gene, says Page, also shows up on the chromosomes of sex-reversed (male) XX mice. To clinch his case, he must now microinject the gene into the embryos of normal female mice.
”If we`re right, they`ll grow into males,” he says.
Great discoveries traditionally have taken place in small laboratories by bright youngsters slaving away in a frenzy of pure science. Page`s fundamental find has sent competitors delightedly dancing around their benches, then dashing off to copy his gene and do him one better, such as finding out what makes the TDF switch on, or fail to.
There is only one problem.
Page has found another TDF gene lurking mysteriously on the female X chromosome. He`s surprised, and now he must explain it.
”We also found it hiding out in all the mammals we`ve studied,” he says. ”It`s probably the most fascinating thing about all this. Since 1959, we`ve thought that sex hinged on the Y. Maybe the X actually does play a part after all.”
There are many possibilities, he says. Perhaps his TDF gene is a fraud. That he doubts. Perhaps the two genes work with, or against, each other to determine sex.
”The possibility I like, because it`s the most outrageous, is that the proteins made by the X and Y genes are identical, and sex is determined by what we call a dosage effect.”
This sounds complicated, but it`s really not. Science has learned that although females have two X chromosomes, they don`t need double doses of the genes found on the Xs.
To avoid this, soon after conception nature orders one X chromosome to shut off the other. This leaves a developing baby girl with only one copy of the TDF gene.
”Males, though, still have two copies, one on each sex chromosome. Sex, thus, may hinge on the total number of genes that are expressed.”
Other scientists tell Page this is much too subtle. The decision is far too important for nature to work this way.
”Except it`s precisely what happens in fruit flies,” says Page, happily.
”Sex in flies, remember, is determined by the number of X chromsomes. Females have two, males only one. So the two-to-one dosage is in effect. The principle may be more general than we had anticipated.”
No matter how sophisticated scientists become, they keep going back to fruit flies.
Page says he can not forsee the TDF gene being used to predetermine the sex of children-”at least not in the near future. It doesn`t make sense to try to apply this in such a way.
”Like everyone, I`m intrigued by the purely philosophical question of what it means to be a man or a woman,” he says. ”In the years that we began to home in on this gene, I found myself becoming even more conscious of how pervasive sexual dimorphism is in our society.
”The public obsession with sex underlies something much more basic-the awesome influence of gender on all aspects of culture. To think that the underpinning might stem from just this single gene is mind-boggling to me.
”But,” he stresses, ”the development of sexual differences in humans is an extremely long and complex process.
”The psychosocial aspects-including gender identity and sexual preference-lie extremely far down the line. If we`re now able to look at the connection between points A and B, those issues probably occur somewhere down around W and X.
”Yet finding this gene, and its partner on the X chromosome, are the kind of discoveries that shape our understanding of how a crucial and complicated process works in the creation of life.”
He expects to devote his entire career to these two little sex chromosomes, piecing together the story, point by point.
”I suspect that right now we`re just starting to chip the ice of something even more exciting,” he says.
