2n−1 is an example of monosomy. It is an aneuploid, in which a chromosome is deficient of its homologous causing genetic imbalance resulting in weaker than normal form. It is formed by the union of deficient gamete (n−1) and normal gamete. A same chromosome monosomic in human beings is Turners syndrome (44+XO=45).
Trisomy is an aneuploid or heteroploid, which has one chromosome represented in triplicate (2n+1). It is formed by the union of a normal gametes with a gamete having an extra chromosome (n+1), eg, Klinefelters syndrome, which results due to additional X-chromosome.
Euploidy is the condition, in which chromosome number is exact multiple of genome like monoploidy, diploidy, polyploidy.