When does trisomy occur during meiosis?

If a gamete with two copies of the chromosome combines with a normal gamete during fertilization, the result is trisomy; if a gamete with no copies of the chromosomes combines with a normal gamete during fertilization, the result is monosomy.

How do most trisomies occur?

The majority of Trisomies are created during Meiosis. Either the sperm or the egg has one extra chromosome. Therefore the embryo inherits 47 chromosomes, instead of the usual 46. As the embryo divides, each cell in the new baby will also contain an extra chromsome.