Todd Henson demands:

Care to show any that sexes evolved, or that such a process could even occur?

Science answers:

Thursday October 28 1999 8:00 PM ET

Chromosome History Traced

WASHINGTON (AP) - The difference between women and men all started 300 million years ago when an ordinary chromosome took the first evolutionary step toward the X and Y chromosomes that now determine the gender of humans, researchers say.

Bruce Lahn of the University of Chicago and Dr. David Page of the Whitehead Institute report that they traced the mutation history of the X and Y chromosomes to a common chromosome that started changing long before humans came along.

The researchers traced the history of the gender genes by reconstructing mutations that make the X chromosome different from the Y chromosome. From this, they were able to estimate when the chromosomes were last identical, some 240 million to 320 million years ago.

``The farther back in time we look, the more similar X and Y appear, boosting the theory that they arose from a pair of identical autosomes,'' said Lahn in a statement. An autosome is a non-sex determining chromosome.

The X and Y chromosomes decide whether a baby is boy or girl. Mothers carry two X chromosomes and fathers carry one copy each of the X and the Y. Sperm from the father has only half of the normal 23 pairs of chromosomes and carries either a single X or Y chromosome. The mother's egg also has half the normal complement of chromosomes and includes a single X. If a Y chromosome from the sperm fertilizes the egg, then the baby is male because the resulting cells each have one Y and one X. If the sperm has an X chromosome, then the child is female, with two X chromosomes in the cells.

Lahn and Page did their research to find how this elegant solution to gender determination evolved.

In many cold-blooded reptiles, which preceded mammals in evolution history, sex of the offspring is determined by the egg incubation temperature. But the development of mammals, which are warm-blooded, required another sex determination system.

Lahn and Page found evidence that early in mammal history, a pair of chromosomes started evolving into the sex chromosomes. A mutation somehow created a gene called SRY - for sex-determining region Y - and it became the master switch for creating a male, they said.

``The SRY-bearing chromosome became the Y chromosome and its SRY-deficient partner became the X chromosome,'' Lahn said.

Over evolutionary time, the male-determining Y chromosome became simpler and shared fewer and fewer genes of the more complex X chromosome. The Y now has only about a tenth of the genes that are present in the X chromosome, the researchers said.

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