Delaying fatherhood may offer survival advantages, say US scientists who have found children with older fathers and grandfathers appear to be "genetically programmed" to live longer.The genetic make-up of sperm changes as a man ages and develops DNA code that favours a longer life - a trait he then passes to his children.The team found the link after analysing the DNA of 1,779 young adults.Their work appears in Proceedings of the National Academy of Sciences. Shoe lace tipsExperts have known for some time that lifespan is linked to the length of structures known as telomeres that sit at the end of the chromosomes that house our genetic code, DNA. Generally, a shorter telomere length means a shorter life expectancy.Like the plastic tips on shoelaces, telomeres protect chromosomal ends from damage. But in most cells, they shorten with age until the cells are no longer able to replicate.However, scientist have discovered that in sperm, telomeres lengthen with age.And since men pass on their DNA to their children via sperm, these long telomeres can be inherited by the next generation.Dr Dan Eisenberg and colleagues from the Department of Anthropology at Northwestern University studied telomere inheritance in a group of young people living in the Philippines.Telomeres, measured in blood samples, were longer in individuals whose father's were older when they were born.The telomere lengthening seen with each year that the men delayed fatherhood was equal to the yearly shortening of telomere length that occurs in middle-aged adults.Telomere lengthening was even greater if the child's paternal grandfather had also been older when he became a father.Although delaying fatherhood increases the risk of miscarriage, the researchers believe there may be long-term health benefits.Inheriting longer telomeres will be particularly beneficial for tissues and biological functions that involve rapid cell growth and turnover - such as the immune system, gut and skin - the scientists believe.And it could have significant implications for general population health."As paternal ancestors delay reproduction, longer telomere length will be passed to offspring, which could allow life span to be extended as populations survive to reproduce at older ages."Prof Thomas von Zglinicki, an expert in cellular ageing at Newcastle University, said more research is needed."Very few of the studies that linked telomere length to health in late life have studied the impact, if any, of paternal age. It is still completely unclear whether telomere length at conception (or birth) or rate of telomere loss with age is more important for age-related morbidity and mortality risk in humans."The authors did not examine health status in the first generation offspring."It might be possible that the advantage of receiving long telomeres from an old father is more than offset by the disadvantage of higher levels of general DNA damage and mutations in sperm, he said.