X Chromosome Marks the Spot

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It's a beautifully simple idea. Since sperm bearing a Y chromosome (the one that creates little boys) contains nearly 3 percent less DNA than its female X chromosome counterpart, why not sort sperm by its genetic weight -- and stack the deck for couples who want to choose the sex of their child? Easier said than done, of course. But that's precisely the technique that a Fairfax, Va., fertility center is set to reveal Wednesday in the journal Human Reproduction. Based on the information released so far, this appears to be the most reliable gender-selection process ever developed.

The technique perfected by the not-for-profit Genetics & IVF Institute involves staining sperm cells with a DNA-sensitive fluorescent dye and then passing them single file through a laser beam. The cells are sorted based on the amount of light reflected by the fluorescent DNA. It's painstaking work -- sorting a single batch takes the better part of a day -- but the results speak for themselves: Of 14 pregnancies produced so far for couples who wanted a girl, 13 fetuses were female. Not entirely foolproof, but pretty close.