In his latest foray into science made sexy, David Bainbridge tells the story of “a little nugget of life called the X chromosome.” He begins with a brief history of human speculation about the origins of sexuality and how this tiny bit of human programming is so essential to continuance of life. “Sexuality was a necessary obsession in the ancient world,” notes the author. From the ancient Greeks to the scientists of the modern world, there has always been interest in the how and the why of the differences and origin of the sexes. Aristotle speculated on the opposition of the sexes but was not sure whether that distinction occurred at the moment of conception. Empedocles believed the early embryo to be sexless until its environment determined whether a boy or girl would result. For centuries the answers to questions sexual would continue to puzzle scientists. Many of the insights proffered by the ancients have turned out to have some grain of truth in them, as Bainbridge illustrates later in the discussion.
One of the most important breakthroughs in the field of genetics occurred in the late nineteenth century. Looking into his microscope in 1890, Herman Henking saw for the first time a collection of tiny flecks of purple (stained chromosomal material) which appeared to him to behave in an antisocial way relative to their confreres. What he had discovered as he observed the division of cells in the testicles of a fly, he would name the “wallflower chromosome.” Because this particular bit of material did not join in the dance in which other chromosomes participated as they became germ cells (sperm or ova), Henking called it “X.” Perhaps the name came from its perceived redundancy.
Henking’s thoughts on the topic do not survive. Rather than redundant, this “X” chromosome would turn out to be very special, the basis of human life. While it has been determined that the Y chromosome has the power to dictate human maleness, it is the X chromosome that is the fallback design, or what might be termed the “natural” human state. The Y seems to have one primary function, but the X is much more. Rather than the woman being a “misbegotten male,” as historical alliteration had suggested, it is the male animal that is nature’s second thought.
Although Bainbridge’s main “character” in the book is the X chromosome, carrying its genetic repertoire with competence, it is the presence and influence of the bit player, the Y chromosome, whose one important spoken line determines the maleness of a particular offspring. This “standard mammal system” of Y’s role holds, even in the aberrant situations in which offspring have additional X chromosomes. Y “wins,” says the author, no matter how many extra X chromosomes nature includes in its genetic instruction manual. Spunk triumphs over numbers. It is a very specific gene, called Sry and carried on the Y, that will consistently determine that a male is produced and the “raw material” primed for femaleness will not become activated. It is Y’s “macho power” which triggers a variety of male-producing chemical activity, sometimes from genes not located on the Y, that makes boys boys.
Having distinct sexes and specific chromosomes which determine them is a fairly recent adaptation in evolutionary history. The chromosomes which are known to function in this manner are likely the result of bits and pieces of other chromosomes being, as Bainbridge puts it, “cobbled together from non-sex chromosomes” over the eons of time. Many creatures still reproduce quite successfully without any sexual determination. There are those who bud, those who simply divide in two, and those who successfully endure and increase by any number of nonsexual processes. Even species with two distinct sexes do not always arrive in that state, as their mammalian cousins do. Many species’ approach to making the male and female depends on external factors rather than the presence of specific X and Y determinants. Some factors which force sexual differences include the condition of the environment (for certain wormlike sea creatures), the temperature of the sand in which the...
(The entire section is 1698 words.)