|
 Half
a century ago, scientists were blaming the Y chromosome for
genes causing hairy ears and skin covered with thousands of
warty bumps ("porcupine skin"). Later, researchers suspected
that the Y carried no genes at all. And even after they realized
that the Y determines maleness ("the Y makes the guy"), for
years they assumed that was the Y's only function. Finally,
the Human Genome Project rescued this unique chromosome from
near oblivion.
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Shared
Pairs
Every
cell in the human body has 46 chromosomes: 23 from the mother,
23 from the father, all of which are long strings of DNA.
Forty-four of these chromosomes occur in essentially identical
pairs; that is, each chromosome in the pair (one from each
parent) carries instructions for the same biological tasks.
Scientists call these 22 pairs "autosomal" chromosomes.
But the last pair, known as the "sex chromosomes," is clearly
different. While women have two X chromosomes- one from mom,
one from dad - men, on the other hand, have one X from mom
and one Y chromosome from dad. While nobody has found genes
for the inability to ask directions or for addiction to the
sports channel on the Y, gene-mapping research has revealed
the true importance of this long misunderstood chromosome.
A
Rotten X?
 |
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| All
mammals, but not all animals, share the X/Y sex determining
system. |
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How
universal is this "Y makes the guy" mechanism of determining
sex? It turns out that all mammals share it: placental mammals
like mice, cats, and dogs; marsupials like opossums and kangaroos;
and even egg laying mammals like duck-billed platypuses. In
birds, however, it's the exact opposite- the Y makes the,
er, hen. Some reptiles have only autosomes and no sex chromosomes
(no X or Y) at all. Instead, other factors determine their
sex. For example, the sex of a baby alligator depends on how
warm it was inside the egg.
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|
While nobody has found genes for the inability to ask
directions, gene-mapping has revealed the true importance
of this long misunderstood chromosome.
|
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If
our mammalian X and Y system is not universal, how did it
come about? It turns out, X and Y chromosomes do share a few
genes. Comparison of such Y genes to their X counterparts
suggests that some 300 million years ago in a reptilian ancestor
of mammals, the X and the Y were still identical twins like
any other autosomal pair. Then, somehow one of these ancestral
chromosomes acquired a gene that determined maleness. That
chromosome was the ancestor of the mammalian Y as we know
it today, while the other copy became the mammalian X.
 |
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| Every
egg contains an X. Sperm determine sex by contributing
either an X or a Y. |
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Autosomal
pairs frequently engage in an exchange- or "recombination"-
of genes with their partner chromosome. But over millions
of years, most of the Y chromosome stopped this exchange with
the partner X, leaving the Y chromosome vulnerable to damaging
mutations and gene loss. In some ways, then, the Y is a shrunken-
but still necessary- form of the X. Some of the genes on the
Y are the last remnants of the many genes shared by the ancestral
X/Y pair.
Occasionally,
an embryo is conceived with only a single X chromosome, and
more than 99% of these embryos die before birth. From this,
we can tell that some of the genes still shared by the X and
the Y must be important: an individual needs two copies of
them to survive. These genes seem to code for fundamental
cellular "housekeeping" processes preserved on the Y precisely
because they are vital to embryonic survival. Indeed, the
Human Genome Project has found more than a dozen such genes
on the Y chromosome.
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pages: | 1 | 2 | 3
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Photos:
University of Utah Andrology Microscopy Lab, Exoticmeat.com,
Advanced Fertility Services, PC
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