Molecular geneticists at Imperial College have provided a clue to what goes wrong when babies with Down's syndrome are born with heart defects.
Nearly half of all babies with the syndrome are born with a heart problem.
The results of the study, published in the Annals of Human Genetics, suggest that a gene on chromosome 21, the chromosome involved in Down's syndrome, may contribute to the cause of the heart defect.
The research team, which is now trying to clone the gene, forms part of the newly established unit of medical and community genetics in St Mary's Hospital medical school at Imperial.
The unit is headed by Anna Kessling, professor of community genetics, who led the genetic study of 58 European families of children with Down's syndrome, and more than 100 people unaffected by it.
Professor Kessling commented: "Finding a gene that seems to be involved in heart defects in children with Down's syndrome may help us understand why they happen, why similar heart defects can arise in some children who don't have Down's syndrome, and how the development of the heart is controlled by the body.
"Once we understand how and why the heart defects happen, we can begin research to work out whether there might be any simple, acceptable way to prevent them."
Children with Down's syndrome have three chromosomes 21. The Imperial College study, supported by more than Pounds 125,000 from the British Heart Foundation, identified how these chromasomes were passed on from the parents, who have two each. It looked at the differences in DNA between parents and children, and between children with and without heart defects.
Professor Kessling said: "In one particular gene, the gene which makes a special type of collagen, a protein found in the lining of the heart, we found evidence of some differences between the 'gene versions' parents gave to children with a heart defect and the 'versions' given to children without a heart defect."
The study suggests the versions given by both parents seem to determine whether or not a child with Down's syndrome develops a heart defect, although the researchers have not yet pinpointed the difference that is having this effect.
Professor Kessling said: "The parents have nothing wrong with them and have not done anything to cause the differences. We think that if a child gets three copies of this particular gene, then the 'versions' the child receives determine whether or not he or she will have a heart defect.
"It may be that some versions of the gene make collagen molecules fit well together in threes, while other combinations are unstable or awkward."