how does downs syndrome happen?

Question

what causes it?

Answer 1

Down syndrome or trisomy 21 is a genetic disorder caused by the presence of all or part of an extra 21st chromosome. It is named after John Langdon Down, the British doctor who described it in 1866.

Answer 2

There are many speculations, but no confirmed cause. Some say it is a reaction to the mercury in the older type innoculations given to kids. Others say it's genetic. They don't really have an answer.