trisomy
Trisomy is having three copies of a particular chromosome where there should be only two. Picture a shelf meant to hold matched pairs of books: trisomy is the shelf that, for one specific title, holds three identical volumes. The cell now carries an extra dose of every gene on that chromosome, and that imbalance is what causes trouble.
Trisomy is a specific kind of aneuploidy and usually results from nondisjunction during meiosis, when a chromosome pair fails to separate so a gamete carries two copies instead of one. After fertilization the embryo then has three. The risk of meiotic nondisjunction rises with maternal age, which is why the chance of some trisomies increases for older mothers.
Whether a trisomy is survivable depends heavily on which chromosome is tripled. Trisomy 21 (Down syndrome) is the most common viable autosomal trisomy; trisomies 13 and 18 are far more severe and usually fatal in infancy. Most other autosomal trisomies end in early miscarriage because the gene-dosage imbalance is incompatible with development.
Trisomy 18 (Edwards syndrome) means three copies of chromosome 18; its severe effects illustrate how the same mechanism can be far less survivable than trisomy 21.
Which chromosome is tripled matters as much as the fact that one is.
Trisomy adds one chromosome (three copies); monosomy removes one (a single copy). Both are aneuploidies, but in opposite directions.