Many an overweight person has claimed that the extra poundage is all due to their genes. There might be something to such an assertion. Researchers studying mice discovered a genetic mutation in the MRAP2 gene that didn't allow them to burn off calories from fat; this same genetic mutation was found in obese humans. Initially, the mice's genetic mutation caused them to eat less than the norm. However, despite the reduced calories, the mice gained about twice the poundage of normal mice. Eventually the mice's appetites returned, and they ate a typical diet — yet still gained more weight than mice without that mutation. Why? The bodies of the mice with the genetic mutation were storing fat, not burning it for fuel. In a study of 500 obese people, scientists found a similar situation with the human version of the MRAP2 gene [source: Sifferlin].
Keep in mind, though, that scientists have identified only about eight genetic mutations that cause obesity in humans. And these mutations are considered to cause less than 5 percent of all of the cases of obesity in society [source: Sifferlin]. So while your portly physique may be in your genes, it's more likely just in your jeans.