Describing an organism or cell that has an abnormal number of chromosomes, not a normal multiple of the basic set.
From Greek 'hetero-' (different) + 'ploid' (from 'ploos' meaning fold, referring to chromosome sets). Coined in the early 20th century in genetics when scientists began understanding chromosome numbers.
Down syndrome is a famous example of heteroploidy in humans (three copies of chromosome 21 instead of two), and while it causes challenges, it shows how sensitive our genetic balance really is.
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