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The Gene Hunters

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Why the Y?
By Steve Rozen, PhD, Genetics Researcher, Whitehead Institute

3 pages: | 1 | 2 | 3 |

Image of spermHalf 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?

Photo of Kangaroos  
All mammals, but not all animals, share the X/Y sex determining system.

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.

While nobody has found genes for the inability to ask directions, gene-mapping has revealed the true importance of this long misunderstood chromosome.

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.

Photo of egg and sperm  
Every egg contains an X. Sperm determine sex by contributing either an X or a Y.

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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3 pages: | 1 | 2 | 3 |

Photos: University of Utah Andrology Microscopy Lab,, Advanced Fertility Services, PC

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