The condition that exists when an organism gains or loses one or more chromosomes but not a complete haploid set is known as .

aneuploidy

Explanation:

Aneuploidy is the presence of an abnormal number of chromosomes in a cell of a human, for example a human cell having 45 or 47 chromosomes instead of the usual 46. It does not include a difference of one or more complete sets of chromosomes. A cell with any number of complete chromosome sets is called a euploid cell.

Generally, the aneuploid chromosome set differs from wild type by only one or a small number of chromosomes. Aneuploids can have a chromosome number either greater or smaller than that of the wild type. Aneuploid nomenclature is based on the number of copies of the specific chromosome in the aneuploid state. For example, the aneuploid condition 2n − 1 is called monosomic (meaning “one chromosome”) because only one copy of some specific chromosome is present instead of the usual two found in its diploid progenitor. The aneuploid 2n + 1 is called trisomic,2n − 2 is nullisomic, and n + 1 is disomic.