Why does trisomy and monosomy occur? And why are they so bad on the molecular level?

Because the 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.In general, a monosomic for a particular chromosome is more severely abnormal than is the corresponding trisomic. trisomy 13, 18, 21, Triploidy and Turner syndrome: the 5T's.

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Explanation:

...~  "Your child's perception of death is abnormal and concerning for the developmental age. Your child should be focused on death as a distant event." ~...