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Last Updated: Friday, 13 February, 2004, 01:00 GMT
Science closing in on ageing gene
The X and Y chromosomes
Experts believe they are a little nearer to tracking down a gene which may influence how long you live.

Belgian researchers say it is probably passed from fathers to daughters and mothers to sons on the sex chromosome.

There is some evidence that suggests a reduction over time in the length of key pieces of DNA in the human cell may contribute to the diseases of old age.

The research, in the Lancet, may help uncover the gene that controls this.

The precise reasons why people appear to age at different rates, and why some will die at a younger age, are likely to be highly complex.

However, the relationship between the length of "telomeres" - which lie at the end of strands of DNA called chromosomes inside the nucleus of every cell - and this process, is becoming a little clearer.

Their length decreases every time the cell divides, and there is the suspicion that beyond a certain point, shortening telomeres might have an impact on longevity or the likelihood of developing diseases of old age.

Why the length of these sections are not the only factor governing longevity, other studies have found good reason to suggest a link between the two.

Many experts believe the rate of shortening may be a factor.

Gene hunt

The research team from Leuven University in Belgium are trying to track down genes elsewhere in the make-up of each individual which might play a role in the telomere-shortening process.

Every human has 23 pairs of chromosomes in their cells - 22 of these pairs are the same in men and women, but the makeup of the final pair differs between the sexes.

These "sex" chromosomes come in two forms - "X" and "Y".

Each new baby gets one sex chromosome from each parent, and the combination they receive dictates their gender.

Women have two X chromosomes, while men have an XY combination.

So while a woman will always pass either of her X chromosomes - but always an X - to her child, the father randomly passes either an X or a Y, giving in theory a 50/50 chance of producing a boy or girl.

If the gene or genes influencing telomere length is carried on the sex chromosome, then it should be possible to work this out by comparing the relationship between telomere length between different parents and children.

Family ties

In this case, when this was measured, there was a distinct relationship between lengths in fathers and daughters, and between mothers and sons.

There was no apparent correlation between telomere length between fathers and sons.

This suggests that the key genes might be carried on the X chromosome, they say.

"X-linked inheritance is the most probable explanation," they write.

Professor Tom Kirkwood, an expert in the biology of ageing from the University of Newcastle, told BBC News Online that there could be many reasons behind telomere shortening - not just the action of individual genes, regardless of their location.

He said: "Since all chromosomes have telomeres and these are transmitted from parent to offspring, whatever factor is on the X-chromosome, if one exists, would have to be able to exert a global effect on telomere length.

"It is not at all obvious how this might occur and I don't think the results rule out other mechanisms."

Years of damage

He said a crucial factor in the rate of decrease in telomere length appeared to be exposure to "oxidative stress" - cumulative damage caused by molecules called "free radicals".

Professor Kirkwood said: "As to whether the apparent pattern of inheritance means anything in terms of ageing, this is even more speculative at this stage."

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