Explanation

In trisomic condition, diploid organism have extra chromosome represented by the chromosomal formula$2n + 1\$. One of the pairs of chromosomes has an extra member, so that a trivalent may be formed during meiotic prophase, e.g., Down’s syndrome (45+XX or 45+XY), Klinefelter’s syndrome (44 + XXY). In monosomic, diploid organism has one chromosome of a single pair missing with genomic formula $2n - 1$. Monosomics can from two kind of gametes, (n) and (n-1), e.g., Turner’s syndrome (44 + X).