(genetics) The chromosomal variation due to a loss or a gain of one or more chromosomes resulting in the deviation from the normal or the usual number of chromosomes for that species
Improper cell divisions may lead to chromosomal variations. One possible outcome is aneuploidy. Another common chromosomal variation is euploidy. Euploidy is a chromosomal variation involving the entire set of chromosomes in a cell or an organism. In aneuploidy, not all of the chromosomes are involved. It may be due to one or more chromosomes but not the entire set. The chromosomal variation may be caused by a lack of or a gain of a copy from the usual chromosomal number. The different conditions of aneuploidy are nullisomy (2N-2), monosomy (2N-1), trisomy (2N+1), and tetrasomy (2N+2). The suffix –somy is used rather than –ploidy. This is essential so that confusion may be avoided between the different types of aneuploidy and the types of euploidy (e.g. monoploidy, diploidy, triploidy, tetraploidy, etc). Furthermore, karyotyping of an aneuploidy (i.e. a cell or an organism displaying aneuploidy) would not be a multiple of the haploid or basic chromosomal number as seen in euploidy.
As an example, the human somatic cell usually has a chromosomal number of 46. In aneuploidy, the chromosomal number may become 45 or 47. One possible cause of the gain or loss of chromosome is when a chromosome does not separate properly (nondisjunction) between the two cells during cell division.