BOSTON — The body’s sex trigger, the elusive gene that determines whether an egg will grow to be a boy or a girl, may have been found at last by scientists.

The gene is a switch that starts an embryo down the path to sexual development after eight weeks in the womb. Those who inherit the gene from their parents become male; those who don’t, become female.

Many scientists, though not all, believe that such a single master sex gene exists. But its precise location and nature have remained a mystery despite intense searching by several rival teams of researchers.

The search competition seemed to be finished in 1987 when Dr. David Page of the Whitehead Institute in Cambridge, Mass., announced the apparent discovery of the gene, which he called ZFY. However, last December, a rival team from the Imperial Cancer Research Fund in London shot down the theory with strong evidence that Page’s discovery is not the sex trigger, at least not all by itself.

Now, the London group has put forth its own nominee. In Thursday’s issue of the British journal Nature, they announced the discovery of “sex-determining region Y,” or SRY.