Always hungry.... Found this article about Prader-Willi syndrome where people have a hard time controlling their eating habits. Check out this snippet:

Prader-Willi could prove a guide to this uncertain terrain because its origins are so specific. Unlike most genetic diseases, the syndrome is rarely inherited. Instead, it is caused by a random accident during egg, sperm, or embryonic formation that either deletes or muffles dozens of genes along a stretch of chromosome 15. Ten of the genes, when disrupted, have been linked to the characteristics of Prader-Willi. In addition to ravenous appetites, Prader-Willi patients have weak muscles, slow metabolisms, small hands, feet, and genitals, and a distinctive triangular mouth. They are often short and very fair, and they tend to have significant learning disabilities. Compulsive behaviors—skin picking, repetitive questioning, and a need to collect and rearrange objects—are common. They can also be very stubborn. (Melissa, luckily, has milder symptoms than most.) Many also have remarkably good memories, and some have an unusual talent with jigsaw puzzles.

As the article makes clear-this syndrome is not inherited. But, it makes a case for causation between a finite number of genes and behavioral trends. In narrow circumstances, ascribing behavior to genes is fine (usually with diseases). Ascribing "normal" behavior to genes seems to be more controversial.