When a child is born with a rare disorder that few doctors recognize or know how to manage, it can pay big dividends for parents to be proactive, learn everything they can about the condition, and with expert medical guidance, come up with the best way to treat it.

That is the approach Lara C. Pullen of Chicago adopted when her son, Kian Tan, was born 15 years ago last month at 7½ pounds, seemingly well-formed and healthy. But within 24 hours, Dr. Pullen, who already had two daughters, said Kian had stopped moving, wouldn’t nurse and felt as floppy as a rag doll.

Two and a half weeks later, a genetic test showed that Kian had Prader-Willi syndrome, a genetic disorder that occurs once in every 15,000 to 25,000 live births. While at first it is a struggle to get enough food into these babies because they’re too weak to suck, within two or three years their main symptom is an insatiable appetite that results in extreme obesity unless the child, who is driven by constant hunger, is kept from sneaking and stealing food.

Prader-Willi syndrome is caused by the failed expression of several genes on chromosome 15 derived from the child’s father. The genes are either missing or inactivated by a mistake that occurs during sperm development or, in some cases, the father’s entire chromosome 15 is not inherited by the fetus. The disorder is only rarely inherited, but when a father has Prader-Willi syndrome caused by a deletion in chromosome 15, there’s a 50 percent chance each child he fathers will inherit the defective chromosome.