Will Chromosome Y Go Bye-Bye?

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Will Chromosome Y Go Bye-Bye?

Is the End of Men Imminent?

By RADHA CHITALE

ABC News Medical Unit

July 17, 2009

What makes a man a man? Socially, that is a complicated question. Genetically,

however, it is as simple as a single Y chromosome.

But guys, that chromosome is in trouble.

In a new study, researchers say there is a dramatic loss of genes from the human

Y chromosome that eventually could lead to its complete disappearance -- in the

next few millennia. While the Y chromosome's degeneration has been known to

geneticists and evolutionary biologists for decades, the study sheds new light

on some of the evolutionary processes that may have contributed to its demise

and posits that, as the degeneration continues, the Y chromosome could disappear

from our genetic repertoire entirely.

" It's certainly possible, but it's difficult to predict when it will happen, "

said Kateryna Makova, an associate professor of biology at Penn State

University, who led the study, which was published Thursday in the journal PLoS

Genetics.

Although geneticists and evolutionary biologists agree that the Y chromosome is

degenerating -- and far more rapidly than its X counterpart -- they reject the

idea of a world far in the future where men are obsolete.

" The idea that the Y chromosome has just bailed out of an airplane without a

parachute simply doesn't fit the facts, " said Dr. Page, director of the

Whitehead Institute for Biomedical Research in Cambridge, Mass., and a Y

chromosome expert. " The evidence from studies on natural deletions of [genes on]

the human Y chromosome shows there are consequences, especially for sperm

production, that implies very strong natural selection against the loss of genes

on the human Y chromosome. "

Y Chromosomes Had Problems From the Start

Humans have 23 pairs of chromosomes packed with genes that dictate every aspect

of our biological functioning. Of these pairs, the sex chromosomes are

different; women have two X chromosomes and men have an X and a Y chromosome.

The Y chromosome contains essential blueprints for the male reproductive system,

in particular those for sperm development.

But the Y chromosome, which once contained as many genes as the X chromosome,

has deteriorated over time and now contains less than 80 functional genes

compared to its partner, which contains more than 1,000 genes. Geneticists and

evolutionary biologists determined that the Y chromosome's deterioration is due

to accumulated mutations, deletions and anomalies that have nowhere to go

because the chromosome doesn't swap genes with the X chromosome like every other

chromosomal pair in our cells do.

However, , lead author of the study and graduate research fellow

at Penn State University, pointed out that if there is no difference between a

male who has lost a particular gene and one who still retains it, especially if

both are still fertile, then that gene must be nonessential.

" Because they can lose [a gene] ... we conclude that it's on its way to dying in

humans, " she said.

Yet the Y chromosome perseveres, despite its rapid rate of deterioration.

" The key flaw in the logic [of Y chromosome deterioration] is the assumption

that the Y chromosome can only lose genes, " Page said. " But the human Y

chromosome has gained genes not even on the X chromosome. Men who lose those

genes do not transmit their Y chromosome. "

Y Chromosome Can Gain Genes

Page pointed out that, while the Y chromosome may not share genetic material

with the X chromosome, it can swap genes with other chromosomes as well as keep

multiple copies of functional genes to increase their number on the Y

chromosome. Makova and said that the increased rate of mutation on the Y

chromosome could give rise to new genes that may prove beneficial and,

therefore, remain on the chromosome.

Genetic change, whether by mutation, environmental stressors or by swapping bits

of chromosomes, is the natural course of evolution, and evolution is weighted

towards survival. Perhaps most importantly, Y chromosomes with defective

male-specific genes, especially those involved in sperm production, are unlikely

to reproduce and pass on those genes to their sons, which knocks highly

defective chromosomes out of the gene pool. Genetic changes that do not favor

reproduction are likely to get weeded out of the system.

" The most fundamental [principle] to all evolution is reproduction, " said Dr.

Crystal, chairman of the Department of Genetic Medicine at Weil Cornell

Medical College. " No one knows why the Y chromosome has more pressures to

evolve. It may be that the genes are irrelevant. ... But evolution figures out a

way to maintain reproduction. "

Reproduction Is Still Paramount for Evolution

Even if the Y chromosome becomes obsolete, reproduction will continue, in some

form. Makova and said that new sex chromosomes may rise from non-sex

chromosomes or that essential genes might move to other chromosomes, which has

happened in some species of deer. " Presumably, we will have moved genes around, "

said Dr. Harry Ostrer, director of the Human Genetics program at the New York

University School of Medicine. " But the reproductive structures will be well

conserved. "

In other words, men will not fade away, even if their Y chromosomes do.