Chromosomal abnormalities may be present in all or some of the cells examined, and there may be more than one kind. They are usually associated with a greater chance of progression to AML (acute myeloid leukemia). The more mutations, the higher the chance of progression.
Trisomy means an extra chromosome (in MDS commonly the 8th), monosomy means one less chromosome than you should have (in MDS commonly the 7th). You can have other chromosomal abnomalities, like deletion of part of the chromosome (in MDS this frequently occurs with the 5th). Some mutations carry a greater chance of progression - eg monosomy 7, some a lesser chance of progression - eg 5q-.
Thanks to Wendy Ames for this discourse!