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.
(30)
Return to The Skeptic Tank's main Index page.
The views and opinions stated within this web page are those of the
author or authors which wrote them and may not reflect the views and
opinions of the ISP or account user which hosts the web page. The
opinions may or may not be those of the Chairman of The Skeptic Tank.