Generate text that supports or refutes this claim: Obesity is determined solely by environmental factors.
Genetics of obesity in adult adoptees and their biological siblings.
An adoption study of genetic effects on obesity in adulthood was carried out in which adoptees separated from their natural parents very early in life were compared with their biological full and half siblings reared by their natural parents. The adoptees represented four groups who by sampling from a larger population were categorised as either thin, medium weight, overweight, or obese. Weight and height were obtained for 115 full siblings of 57 adoptees and for 850 half siblings of 341 adoptees. In full siblings body mass index (kg/m2) significantly increased with weight of the adoptees. Body mass index of the half siblings showed a steady but weaker increase across the four weight groups of adoptees. There were no significant interactions with sex of the adoptees, sex of the siblings, or (for the half siblings) sex of the common parent. In contrast with the findings in half siblings and (previously) the natural parents there was a striking, significant increase in body mass index between full siblings of overweight and obese adoptees. The degree of fatness in adults living in the same environment appears to be influenced by genetic factors independent of sex, which may include polygenic as well as major gene effects on obesity.