DNA clue to mental handicap

Scientists believe they may have found the underlying genetic defect responsible for a significant proportion of children born mentally handicapped, writes Steve Connor. The researchers have discovered "cryptic" rearrangements of genetic material at the tips of chromosomes which they think can result in congenital mental defects.

About 3 per cent of children are born mentally retarded and about half of these cases cannot be explained.

Scientists at the Institute of Molecular Medicine at Oxford University have evidence that a significant proportion of these unexplained cases are due to sub-microscopic swaps of DNA - the genetic blueprint - at the tips of the chromosomes which can go unnoticed by conventional microscopic analysis.

Andrew Wilkie, a member of the research team, said that chromosome tips appear to be unusually vulnerable to errors during the manufacture of sperm and eggs, which can result in genetic defects being passed on after conception.