aneuploidy /an″yo͝oploi′dē/ , 1. any variation in chromosome number that involves individual chromosomes rather than entire sets of chromosomes. There may be fewer chromosomes, as in Turner’s syndrome (one X chromosome in females), or more chromosomes, as in Down syndrome (three copies of chromosome 21). Such individuals have various abnormal physiological and morphological traits. Compare euploidy. See also chromosomal aberration, monosomy, trisomy. 2. a phenomenon when the number of chromosomes in an individual, cell, or strain is not an exact multiple of haploid number characteristic of the species.
