Monosomy (the lack of one member of a chromosome pair) and trisomy (a triplet instead of the normal chromosome pair) are typically the result of nondisjunction during meiosis. When this happens, one gamete shows monosomy, and the other shows trisomy of the same chromosome.

When does monosomy occur in 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 does partial monosomy occur?

Partial monosomy happens when part of a chromosome is missing because of deletion or chromosome translocation. Monosomy causes major problems because our cells are optimized to have two copies of each gene, so the majority of monosomic embryos die early in development.