Chromosomes make the man
The literature of science offers no more brazen invitation than the opening paragraphs of a new book on the molecular biology of sex by London geneticist Steve Jones.
“Ejaculate, if you are so minded and equipped, into a glass of chilled Perrier,” he begins. “There you will see a formless object, but look hard enough -- or at least so eighteenth-century biologists believed -- and a baby appears: the male’s gift to the female, whose only job is to incubate the child produced with so much labor by her mate. So central seemed a husband’s role that his wife was a mere seedbed, a step below him in society, in the household, and, most of all, in herself. Foolish of course, and quite wrong, for biology proves that man, and not woman, is the second sex.”
With just so many words, Jones limns a revolution in human sexual biology that mirrors an equal transformation in the political and social understanding of the relations between men and women. In “Y: The Descent of Men,” Jones explores this changing state of masculinity with considerable relish by deconstructing the curious genetics of the male Y chromosome.
This is a sexual and scientific revolution still in progress. Indeed, Jones writes so near the cutting edge of research that at times his insights have been overtaken by scientific findings made public in the months since his book was published. In this sense, Jones has been sideswiped by the speeding juggernaut of science -- an occupational hazard for those authors who seek to explicate the esoterica of experimental research to the general public.
Any flaw in the timeliness of the scientific findings he reports in no way diminishes the deeper pleasure of this book. In seeking to understand the biological question mark known as the male of the species, Jones plumbs more enduring truths of human sexuality.
For the real question of biology, Jones suggests, is why men? And why so many?
In the beginning, there was no sex -- at least not in the way we normally conceive it. Gender was triggered not by the internal workings of genes and chromosomes but by an arbitrary external influence, such as temperature. For many species, that is still the meaning of hot sex. The difference between a male and female crocodile or sea turtle is a thermometer reading as the egg is being incubated.
Some 300 million years ago, a new mechanism of sexual identity evolved, a biochemical blueprint from which all the sorrows of sexual misunderstanding can be traced. In this method, females have a pair of X chromosomes, as every biology student learns, while males have only one X and a Y that tips an embryo down the developmental pathway of maleness.
The Y chromosome is the catalyst that makes sex possible. With it, human reproduction becomes the random lottery of evolution, in which genes are constantly mixed in new combinations to guarantee that every child is unique. “This recombination means that sex, not death, is the great leveler,” Jones writes.
These male and female sex chromosomes began on an equal footing. Indeed they had so much in common as to be virtually indistinguishable. Today, however, they are worlds apart. The male appears the poorer for it. The Y chromosome may have had as many as 1,000 genes when it began; today it has fewer than 100.
The Y may be the smallest of the 46 chromosomes that control human heredity, yet this dangling biochemical participle may have done more than any other part of the human genome to shape how people view the world and their place in it. The runt of the genetic litter is at the heart of the profound biological differences between men and women in anatomy, physiology, cognition, behavior and vulnerability to disease.
This fragment of DNA passes from every father to every son as a biochemical heirloom of male lineage. The Y chromosome is an arrow of manhood, Jones writes, that flies from Adam to every male alive today.
However, for Jones -- and the generation of geneticists on whose work he draws -- the scientific essence of masculinity is a stunted object of desire that has been ruined, oddly enough, by celibacy. Most of the Y chromosome, it turns out, does not participate in the swapping of genes during the creation of egg and sperm. At best, it is merely a conduit through which move genes from the real masters of the human universe -- women.
On its face, the statistical particulars are puny. The Y comprises just a fraction of the total genome, containing only 60 million of the 3.3 billion base pairs of DNA in the human genetic sequence. And most of that is made up of genetic misprints -- typographical errors in the language of life. To all appearances, the male chromosome is a genetic junkyard, Jones writes.
Its status as the master switch of maleness also may be overstated. Two genes -- one from the female X chromosome and one from the male Y -- guide the formation of normal testicles. Many of the genes needed for sperm production reside on the female chromosome, not the male.
In his effort to set the sexual record straight, Jones brings to bear his considerable scientific expertise as a geneticist at the Galton Laboratory, one of the oldest world centers for genetic research. His drollery enlivens erudite digressions on the hydraulics of erections, flagging sperm counts, peacock plumage, urinals, phallus collections and castrated opera singers.
At his best, he is the Seinfeld of science writing. An arched eyebrow hovers over every male oddity. Who would have imagined that fruit flies could produce a single sperm that uncurled is as long as a human finger, or that pigs produce pints of seminal fluid? Every time a man has sex, he produces enough sperm to impregnate every woman in Europe. The chemical precursors of laughing gas, Jones reminds us, inflate every erection.
With such relish, Jones demolishes shibboleths of male superiority. In the process, he risks embracing too eagerly new cliches of male sexual stereotypes. At this molecular and cellular level, men are sexual simpletons, while women are adept multi-taskers who enfold and nurture and empower. “Males are, in many ways, parasites upon their partners,” Jones writes. What working woman with children would argue?
The title of his book mocks the 1871 classic text of evolution -- “The Descent of Man and Selection in Relation to Sex” by Charles Darwin. Darwin may have been right about the role of natural selection in the quest for evolutionary advantage, but he was mostly wrong about sex. His ideas about the natural order of ravaging males and shy, submissive females were relentlessly Victorian. In Jones’ view, Gilbert & Sullivan were more on the mark. He approvingly quotes the lyrics from one of their more obscure operettas: “Man is coarse and Man is plain; Man is more or less insane; Man’s a ribald; Man’s a rake; Man is Nature’s sole mistake.”
New research from scientists at the Whitehead Institute for Biomedical Research in Cambridge, Mass., strongly suggests that this view may be a tad simplistic. The Y appears to contain more genes than scientists had thought and seems quite capable of repairing itself without recourse to normal reproduction. Much of what had been labeled its nonsense DNA turns out to have a covert evolutionary purpose.
Researchers discovered that most of its genes are concealed in eight long sentences of DNA characters that read the same forward and backward, the most extensive mirror sequences known in any species. The longest of these Y palindromes, as such reversible sequences are called, is almost 3 million characters long and is 99.7% identical. It is as if Jones’ entire book read the same from the front and from the back. These extensive repetitive gene sequences allow the chromosome to repair and proofread its own DNA.
Rather than decaying as geneticists have argued, the Y chromosome appears to be evolving faster than the human genome as a whole, altering its structure more rapidly than even the most knowledgeable writer can keep pace.
