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THE human Y chromosome, which carries the genes determining male sex, may be
a far closer relative of the X chromosome than anyone thought.

Geneticists believe the Y chromosome evolved from an ancestral X chromosome
which lost most of its genes. But in early mammals, the male sex genes are
thought to have hopped onto the Y from other chromosomes.

Now Jennifer Graves and her colleagues at La Trobe University in Melbourne
have found a copy of a male sex-determining gene called RBM on the X chromosome
of both humans and kangaroos, which diverged from a common ancestor 130 million
years ago. They believe this shows that RBM was already on the ancient X
chromosome when it evolved into the Y. The gene has very little effect in
females.

Graves says she has evidence that some other male sex-determining genes on
the Y also came from the X: 鈥淭hey didn鈥檛 come from somewhere else. They have
always been there.鈥�

鈥淚 think her idea is a good one,鈥� comments Katrin Schiebel, who studies
chromosome evolution at the University of Heidelberg in Germany. 鈥淭here will
probably be more genes like this on the Y.鈥�