Making Sure About Sex

A new test helps erase doubt

Though nature endows most people with sexual organs that are distinctly male or female, every so often it errs. On rare occasions, a child is born with ambiguous genitalia. In some cases, babies who look outwardly female also have rudimentary male organs--or have testes within their ovaries (a growth that frequently becomes cancerous). Other infants, at first appearing to be male, have small testes, are sterile, and later develop breasts and grow no facial hair.

Fortunately, this gender confusion can usually be corrected with sex-hormone therapy and delicate surgery. Because the treatment is seldom reversible, a mistake in picking the proper gender can have disastrous consequences in later life. Thus, doctors are often uncertain about which sexual direction to take, especially with patients who are too young to have developed any sexual identity. Now a sophisticated new test is available to help doctors make that crucial decision. In using the technique, devised by Immunologists Stephen Wachtel, Gloria Koo and their colleagues at Manhattan's Memorial Sloan-Kettering Cancer Center, doctors ignore external sexual features. Instead, they investigate the basic arbiters of sexual identity, the chromosomes.

In each human cell, there are 46 chromosomes, which are actually long protein-wrapped strands of the master molecule DNA, containing thousands of heredity-bearing segments called genes. Half of the chromosomes are inherited from the father, half from the mother--including two sex chromosomes, one called the X, the other the Y. If in the genetic lottery of conception, the fertilized egg happens to get two X chromosomes, it will usually develop into a female. If it gets an X and a Y, it will probably become a male.

Yet sometimes an infant's chromosomal sex will not match its outward appearance. There are, for example, people with a female XX chromosome makeup who display pronounced male sexual development. Wachtel discovered why. Analyzing the blood of these individuals, he found that their chromosomes apparently contained a hidden male factor--a variant of the same signal found in normal males. In the first few weeks after conception, when the embryo is still sexually neutral, the signal causes the development of testes, a source of male hormones. In the case of the XX males, however, the male factor, apparently consisting of a bit of genetic material--possibly no more than a single gene--is somehow attached to a chromosome other than the usual Y. The proof of this aberration came from an immunological experiment; white blood cells from an XX male were mixed --and then reacted--with a test antiserum. That reaction was confirmation of the presence of the so-called H-Y or "male" antigen.

Wachtel, 40, a former Air Force jet instructor-pilot who only started studying immunology in his 30s, does not yet know how the wrong chromosomes acquire the male gene; possibly the error is the result of a mutation or an accidental transfer of genes between chromosomes while the sperm are being produced in the father's testes. In any case, the test has already been put to important clinical use. At New York Hospital-Cornell Medical Center, for example, a two-year-old child with an enlarged clitoris but a seemingly normal XX chromosome pattern was tested for H-Y antigen. The assay turned out positive, and subsequent surgery revealed the reason: she was a hermaphrodite, with dual sexual organs--an ovary on one side and a combination of ovary and testis on the other. The doctors promptly removed the ovotestis, thereby reducing the danger of cancer and ensuring that she would be an unambiguous female.

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