A condition in which not all the chromosomes are present in equal numbers and hence the total number is not an exact multiple of the haploid set. It occurs when chromosomes fail to separate at meiosis (see nondisjunction), so a gamete may either lack one chromosome altogether or have an additional copy. On fertilization the resulting zygote may thus have only one homologue of a given chromosome, and is described as a *monosomic
, or it may have three homologues, and is called a *trisomic
. If both gametes lack the same chromosome then the zygote is said to be nullisomic. Nullisomics are often invi-able. Alternatively if both gametes contain the same additional chromosome, the zygote is termed tetrasomic. If there are missing or additional copies of two chromosomes the zygote is described as double monosomic or double trisomic respectively.
Aneuploidy results in unusual segregation ratios. It has been most closely studied in the thorn apple (Datura stramonium) where 12 types of trisomics have been recognized (one for each of the 12 different chromosomes), each producing a different mutant phenotype.