Like many other medical conditions, obesity is the result of an interplay between genetic and environmental factors. Polymorphisms in variousgenes controlling appetite and metabolism predispose to obesity when sufficient food energy present. As of 2006, more than 41 of these sites on the human genome have been linked to the development of obesity when a favorable environment is present.[99] People with two copies of theFTO gene (fat mass and obesity associated gene) have been found on average to weigh 3–4 kg more and have a 1.67-fold greater risk of obesity compared with those without the risk allele.[100] The percentage of obesity that can be attributed to genetics varies, depending on the population examined, from 6% to 85%.[101]
Obesity is a major feature in several syndromes, such as Prader-Willi syndrome, Bardet-Biedl syndrome, Cohen syndrome, and MOMO syndrome. (The term "non-syndromic obesity" is sometimes used to exclude these conditions.)[102] In people with early-onset severe obesity (defined by an onset before 10 years of age and body mass index over three standard deviations above normal), 7% harbor a single point DNA mutation.[103]
Studies that have focused on inheritance patterns rather than on specific genes have found that 80% of the offspring of two obese parents were also obese, in contrast to less than 10% of the offspring of two parents who were of normal weight.[104]
The thrifty gene hypothesis postulates that, due to dietary scarcity during human evolution, people are prone to obesity. Their ability to take advantage of rare periods of abundance by storing energy as fat would be advantageous during times of varying food availability, and individuals with greater adipose reserves would be more likely to survive famine. This tendency to store fat, however, would be maladaptive in societies with stable food supplies.[105] This theory has received various criticisms, and other evolutionarily-based theories such as the drifty gene hypothesisand the thrifty phenotype hypothesis have also been proposed