what causes a simian crease
Causes of Simian crease that are rare The offical US government definition of a rare disease is one that affects 200,000 people or less. The following causes of Simian crease appear in the population at a rate of less than 200,000 people per year in the USA: Coffin-Lowry syndrome - simian crease.
The simian crease is one of the listed signs in Down Syndrome which would imply there's some genetic factor at play. My eldest daughter has a simian crease on one hand and the other hand is 'normal'.