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Sex and “The Y”
By Kate Dalke

The Y chromosome is what makes guys, well, guys.

Male chromosomes, The X (left) and Y magnified about 10,000 times.

Among chromosomes, the Y is a bit of a celebrity, something a little different. Among scientists, the Y is infamous for containing some of the most complex regions to sequence in the human genome.

Nonetheless, researchers announced this week that 95 percent of the human Y chromosome has been sequenced—a chunk of DNA called the male-specific region. They pinpointed 78 genes, including those that control development of the testes.

They were surprised to find massive palindromes—hairpin-like structures that contain DNA sequences that read the same backward and forward. Palindromes in effect contain backup versions of genes, allowing the chromosome to eliminate harmful mutations.

So what makes guys, well, guys in other animals?

The palindromes on the Y chromosomes of gorillas and chimpanzees are virtually identical to those in humans, but almost nothing is known about the Y in the rest of the animal kingdom.

Scientists have been unable to sequence the Y chromosome in fruit flies, and although there is a female mouse genome, no one has attempted to sequence the male-specific region in mice.

In birds, the sex chromosomes are flip-flopped compared with humans. Women carry XX and gents carry XY, but in birds, females are XY and males are XX.

Things get weirder in reptiles. Sometimes there are no sex chromosomes at all; instead, the sex of offspring is determined by the temperature of eggs during incubation.

Male birds carry XX chromosomes and red-eared slider turtles have no sex chromosomes.

In the red-eared slider turtle, for instance, low temperatures in the nest produce males, while warmer temperatures produce females.

Does this mean that some years there will be more baby Jack turtles than baby Jill turtles?

Not necessarily, says David Crews of the University of Texas in Austin, who studies sex differences in these turtles and other animals. The ratio of males to females in the general population seems to be even.

Meanwhile, the Y chromosome is a tried-and-true system for determining sex in humans, plants, insects, and some fish, says Helen Skaletsky of the Whitehead Institute/MIT Center for Genome Research in Cambridge, Massachusetts, where the Y was sequenced.

“The system is robust, and it doesn’t depend on the weather,” says Skaletsky.

. . .

Skaletsky, H. et al. The male-specific region of the human Y chromosome is a mosaic of discrete sequence classes. Nature 423, 825-837 (June 19, 2003).
Rozen, S. et al. Abundant gene conversion between arms of palindromes in human and ape Y chromosomes. Nature 423, 873-876 (June 19, 2003).

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