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BOB HIRSHON (host):
The when of the Y chromosome. I’m Bob Hirshon and this is Science Update.
For mammals like us, being biologically male is largely determined by having a Y chromosome. Now, for the first time, scientists have traced the Y chromosome’s origin. According to Diego Cortez of the Swiss Institute of Bioinformatics, sex-determining Y genes emerged separately in two different mammal lines, about 180 million years ago. What made them male and female before that is anyone’s guess.
DIEGO CORTEZ (Swiss Institute of Bioinformatics):
We actually don’t know the nature of the sex system in the ancestor of all mammals. That’s still an open question.
HIRSHON:
Possibilities include environmental factors like temperature, which influences sex in modern reptiles. Cortez says clues might be found in a few species of rodents, which actually lost the Y chromosome in their recent evolutionary past, and now develop into one sex or the other through a yet-to-be discovered mechanism. I’m Bob Hirshon for AAAS, the Science Society.