Prader-Willi syndrome is caused by a genetic change on chromosome number 15.

Genes contain the instructions for making a human being. They're made up of DNA and packaged into strands called chromosomes.

A person has 2 copies of all their genes, which means chromosomes come in pairs.

Humans have 46 chromosomes (23 pairs). One of the chromosomes that belongs to pair number 15 is different in Prader-Willi syndrome.

Around 70% of cases of Prader-Willi syndrome are the result of missing genetic information from the copy of chromosome 15 inherited from the father. This is referred to as "paternal deletion".

Paternal deletion is thought to happen entirely by chance, so it's practically unheard of to have more than 1 child with Prader-Willi syndrome caused by paternal deletion.

However, if Prader-Willi syndrome is caused by a different type of change with chromosome 15, there's a very small chance you could have another child with the syndrome.

It's thought the problem with chromosome 15 disrupts the development and functioning of part of the brain called the hypothalamus.

The hypothalamus plays a role in many of the body's functions, such as producing hormones and helping regulate appetite.

A malfunctioning hypothalamus may explain some of the typical features of Prader-Willi syndrome, such as delayed growth and persistent hunger.

Studies using advanced brain imaging technology have shown that after eating, people with Prader-Willi syndrome have very high levels of electrical activity in a part of the brain known as the frontal cortex.

This part of the brain is associated with physical pleasure and feelings of contentment.

It may be possible that people with Prader-Willi syndrome find the act of eating immensely rewarding.

Another theory is that in Prader-Willi syndrome, the hypothalamus cannot judge the level of food in the body as well as it usually does. 

This means that a person with the syndrome always feels hungry no matter how much food they eat.