Image courtesy Donald Levin
In men, the 23rd chromosome is made up of an X chromosome and a Y chromosome. The 22 other chromosomes in human cells are matched pairs, shared by men and women.
In men, the 23rd chromosome is made up of an X chromosome and a Y chromosome. The 22 other chromosomes in human cells are matched pairs, shared by men and women. Image courtesy Donald Levin
It takes a man to carry a Y chromosome, and it takes the Y chromosome to make sperm, which is necessary for human reproduction. So men are essential to the future of the species.
But researchers have found that, over the millennia, the Y chromosome has lost most of it genes. What if it were to disappear altogether? NPR's Joe Palca explores that possibility in the first report of a three-part series on the End of Men.
The Y Chromosome: A Primer
Each of our cells contains 23 pairs of chromosomes. Twenty-two of those pairs are matched pairs, shared by men and women. The 23rd is different.
In women, the 23rd pair is made up of two X chromosomes. In men, it's made up of an X chromosome and a Y chromosome. That Y chromosome determines maleness in humans — it holds genes necessary for forming testes and making sperm.
Y So Lonely?
The fact that it doesn't have a matching pair poses a bit of a problem for the Y chromosome.
All the other chromosomes come in two copies. Every time a cell divides, mistakes in genes can creep in. In paired chromosomes, that means that if there is a mistake on one chromosome, a cell can always get the correct gene sequence from the other chromosome.
Over time, mistakes have crept into the Y chromosome, too. But every time a gene on the Y chromosome went bad, it basically disappeared. Scientists theorize that the X and Y chromosome started out with about the same amount of genes — about 1,000. Today, the Y chromosome has less than 80 genes.
Hope for Y's Future
Some geneticists think the Y chromosome is now little more than a genetic wasteland that will eventually just disappear. If that were to happen, it would certainly spell the end of sexual reproduction.
Thanks to Dan Crall of Indiana Public Radio for recording some of the crowd cheering sounds heard in this story. Crowd sounds were provided by audiences at Ball State University's Freshmen Connections lecture program, and the 2004 A. Dixon and Betty F. Johnson Memorial Lecture in Scientific Communication at the Penn State University Eberly College of Science.
But David Page of MIT's Whitehead Institute vigorously disagrees. "At the same time that it is continuing to lose genes, it's found some new ways of replenishing itself," Page says.
Last year, Page and his colleagues reported a finding that brightened the outlook for the future of men: The Y chromosome has been secretly creating backup copies of its most important genes. These are stored in the DNA as mirror images, or palindromes — which read the same way forwards and backwards. ("Madam, I'm Adam" is a famous example.)
In Y chromosome palindromes, the first half contains the gene and the second half contains the same information, just in reverse.
That means that many of the genes on the Y chromosome do occur as pairs. Page says members of these pairs appear to be swapping out or recombining with each other — allowing the genes to repair themselves when they get damaged.
Page says this helps explain why these genes have been able to persist despite millions of years of assault from random mutations. And, he says, it means the Y chromosome won't simply keep shrinking away until it disappears altogether.