LONDON (Reuters) People with extra copies of certain genes are much more likely to be very skinny, scientists said last week in the first finding of a genetic cause for extreme thinness.In a study in the journal Nature, researchers from Britain's Imperial College London and the University of Lausanne in Switzerland found that a duplication of a part of chromosome 16 is associated with being underweight. Previous research has found that people with a missing copy of these genes are 43 times more likely to be morbidly obese. "This is the first genetic cause of extreme thinness that has been identified," said Philippe Froguel from Imperial's school of public health, who led the study. "It's also the first example of a deletion and a duplication of one part of the genome having opposite effects."He said one reason this latest finding was important is that it shows that failure to thrive in childhood can be genetically driven. "If a child is not eating, it's not necessarily the parents' fault," he said.Normally, each person has a copy of each chromosome from each parent, giving them two copies of each gene. But sometimes sections of a chromosome can be duplicated or deleted, resulting in an abnormal "dosage" of genes, the researchers explained in their study.But in around one in 2,000 people, part of chromosome 16 is duplicated, making men 23 times and women five times more likely to be underweight.