Toward Sex on Order

Can science help parents choose in advance the sex of their baby? The answer: not now, but perhaps before very long.

Biologists believe that a child's sex is determined at conception by the kind of male sperm that enters the female egg. If the sperm carries a Y chromosome, the child will be male. If it carries an X chromosome, it will produce a girl. The female egg has little to do with the boy-or-girl result.

All well and good, except that biologists have always had a hard time telling the difference between an X sperm and a Y sperm. Now, in Nature, Dr. Landrum B. Shettles, of Columbia University's College of Physicians and Surgeons, reports on what may be a simple way to differentiate X from Y.

In his laboratory Shettles spread sperm cells thinly on a glass slide, allowed them to dry, and examined them with a phase-contrast microscope--a type that makes tiny objects look like bright halos of light against a dark background, showing up details that ordinary microscopes miss. As the sperm dried, Shettles found that the heads of some looked round like doughnuts ; others appeared long and boat shaped. There were no intermediate types, although the size of the sperm varied a good deal from sample to sample. Shettles speculates that the roundheads carry the male-producing Y chromosome, while the longheads carry the female-producing X chromosome. In every specimen the roundheads outnumbered the longheads, which checks with the fact that about 105 boys are born to every 100 girls.

The identification of dried X and Y sperm should help scientists learn how to identify living sperm and later to separate them into "males" and "females." Once this is done, parents can, if they want to resort to artificial insemination, decide the sex of their unborn child.

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