Aneuploidy can be either due to loss of one or more chromosomes (hypoploidy) or due to addition of one or more chromosomes to complete chromosome complement (hyperploidy). Hypoploidy is mainly due to loss of a single chromosome monosomy (2n - 1), or due to loss of one pair of chromosomes nullisomy (2n - 2). Similarly, hyperploidy may involve addition of either a single chromosome trisomy (2n + 1) or a pair of chromosomes tetrasomy (2n + 2). In representing chromosome number of aneuploids, here we are using 2n as the euploid chromosome number, even though 2n actually represents the somatic chromosome number of any organism, whether euploid or aneuploid. It is for this reason that in the preceding paragraph aneuploids are shown as 2n = 15 or 2n = 13 and not as 2n + 1 = 15 or 2n - 1 = 13.