The ‘Gay Gene’ and the Politics Surrounding It : A SEPARATE CREATION: The Search for the Biological Origins of Sexual Orientation, by Chandler Burr (Hyperion; $24.95, 354 pages)

“The Twilight of the Golds” is a recent stage play about the agonies of a pregnant woman who ponders whether to abort her baby when a genetic test predicts that the child will be gay.

When the play was written only a few years ago, the notion of testing to determine sexual orientation was not much more than a playwright’s conceit. Today, however, some geneticists believe that they have identified a “gay gene” that plays a role in defining the sexual identity of men and women from the moment of conception.

“By the time the research is complete, we may be able to distinguish in some lab test the differences that make some of us homosexual,” predicts journalist Chandler Burr in “A Separate Creation,” an enlightening new book about the cutting-edge discoveries in genetics and sexuality. “It means that when a woman is pregnant, a few stray cells from her womb may tell her, or someone else, this aspect of her child. It means that gay people will no longer have the option of hiding.”

The earliest revelations of a link between genetics and sexual orientation first reached the mass media back in 1991 and prompted what Burr calls “a fury based on confusion.” His book is an earnest and mostly successful effort to untangle and explain the revolutionary findings of genetic research into the makings of sexual identity.

The latest experiments, as Burr explains in sometimes daunting detail, suggest that sexual preference is determined, at least in part, by genetic chance. In other words, one does not choose to be gay; one is not twisted into being gay by traumatic childhood experiences; rather, it may turn out that one’s sexual proclivities are “hard-wired” at birth.

Burr is a journalist who first reported on the pioneering discoveries about genetics and sexual orientation in “Atlantic Monthly” in 1993, and now he has fleshed out his notes into a book-length study that is, at once, a sizzling medical mystery story and a compelling profile of the sometimes eccentric scientists whose life’s work is the study of rat testicles, the hormonal ups and downs of masculinized female hyenas, and the brain matter of human corpses.

Along the way, Burr explains the convoluted and sometimes downright comic politics of genetic research, the hotly debated scientific significance such arcane topics as of the Nissl stain and the Belgian Wasserslager canary experiments, and the volatile political implications of genetic science, all with good humor and great clarity.

Burr stalwartly refuses to hype his own highly technical subject matter, to lionize his sources, or to novelize what might strike less enterprising writers as a story with rather too much scientific jargon and too many obscure scientific theories. Still, the plain-spoken and matter-of-fact quality of Burr’s reporting leads to moments of understated horror that reveal something important about what is really at stake in the study of human sexuality.

Burr never allows us to forget that the whole subject of human sexuality--and especially gay sexuality--is freighted with moral and even theological overtones, and so is the genetic research into sexual orientation. Significantly, Burr introduces us to one scientist who voiced doubts about a genetic basis for homosexuality--and was promptly solicited by a Christian fundamentalist group to present evidence in court in an effort to defeat the civil rights of gay men and women.

Here we find the not-so-subtle subtext of the scientific research into sexual preference and the book itself--the suggestion that homosexuality is caused or at least influenced by one’s genetic legacy can be used as a weapon for and against gay men and women.

“All human genetics is political,” says one Harvard geneticist, thus summing up the real danger that Burr seeks to warn against in “A Separate Creation.”