Is trisomy 21 caused by nondisjunction in meiosis 1 or 2 ? please state resource?

I occurs during Synapsis, this is when the chromosomes line up in pairs at the center of the cell. When they separate sometimes the chromosomes stick together. This can result in a gamete with missing chromosome and a gamete with an extra chromosome. If a baby results from the lack of the 21 chromosome is is referred to as trisomy 21.