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Aneuploidy exists when the chromosome number is not 46, but not an exact multiple of the haploid number of 23.
The most common causes of aneuploidy are in the meiotic or mitotic formation of gametes - delayed movement of a chromatid at anaphase and non-disjunction of chromosomes in metaphase.
The aetiology of non-disjunction is not known, but it occurs with increasing frequency with: