The prototypic functional gastrointestinal disorder is irritable bowel syndrome (IBS) which affects approximately 10% of the population and causes considerable morbidity.¹ Traditionally, the pathogenesis of IBS has been simplistically conceptualised by applying the biopsychosocial model; this focuses on the interaction, via the brain–gut axis, of psychosocial processes with pertubations of gut sensory and motor dysfunction.² The model suggests that either genetic or environmental factors may alter central and peripheral physiology. However, how this occurs remains mysterious, few environmental factors have been linked to IBS and indeed whether or not genetics plays a part remains controversial.

There are clues that IBS is an inherited condition.^3–5 In a family cluster study of 643 subjects from Olmsted County, Minnesota, those who reported having first degree relatives with bowel problems were significantly more likely to report IBS, adjusting for age, sex and the reporting of non-gastrointestinal somatic symptoms.³ Those who reported having a spouse with bowel problems were no more likely to report IBS.³ A study that directly surveyed relatives confirmed that there was nearly a threefold increase in IBS among the relatives of patients with IBS compared with the relatives of spouses.⁴ However, these results could all reflect a common intrafamilial environment rather than genes.

Two twin studies have suggested that there is a genetic component in IBS but clearly also indicate that the environment is key.^6,7 Twins studies rely on the fact that monozygotic (identical) twins share 100% of their genes while dizygotic twins (like siblings) share only 50%. The first study to directly assess the potential role of genetics in IBS evaluated Australian twins.⁶ We found in this study that there was a …