People with extra copies of certain genes are much more likely to be very skinny, scientists said on Wednesday, in the first finding of a genetic cause for extreme thinness.

In a study in the journal Nature, researchers from Lausanne University and Britain’s Imperial College London 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.

Philippe Froguel from Imperial’s school of public health, who led the study, said one reason this latest finding was important was that it showed that failure to thrive in childhood could 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.

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. Being underweight is defined as having a body mass index (BMI) below 18.5 kg per metre squared.

Froguel said scientists still had much work to do to find out more about the genes in this region, but their discovery could eventually lead to new potential treatments for obesity and appetite disorders.

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