Original
An extra one or two chromosomes. It can be an X or Y. And it can be multiple ones. XXY is the most common with XYY being the next most common. But it includes XXYY, XXXY and XYYY. The more chromosomes the more debilitating the condition is and the more likely one will pass away sooner then later in life. Genetics is definitely the cause. If it was environment or exposure to radiation or exposure to anything we would see population grouping. It occurs in all races, countries and down through the ages of time.