aneuploidy

Aneuploidy is having the wrong number of one or a few chromosomes — like a deck of cards that should hold exactly two of every rank but accidentally has three jacks or only one queen. The total count is off not by a whole extra set, but by one or a couple of individual chromosomes. In humans the normal number is 46 (23 pairs), so 45 or 47 chromosomes is aneuploid.

Aneuploidy usually arises from nondisjunction, a mistake during cell division when a chromosome pair fails to separate properly. One daughter cell then receives an extra copy and another receives too few. When this happens in egg or sperm formation, the resulting embryo carries the wrong number in every cell; when it happens later in development, only some cells are affected, producing mosaicism.

The consequences are usually severe because chromosomes carry hundreds of genes, and the cell is sensitive to the dose of all of them at once. Most autosomal aneuploidies cause miscarriage; a few, such as trisomy 21, are compatible with life but bring characteristic features. Aneuploidy of the sex chromosomes (as in Turner or Klinefelter syndrome) tends to be milder, partly because X-inactivation buffers extra X chromosomes.

Aneuploidy is also a hallmark of cancer cells, which frequently accumulate extra and missing chromosomes as their genomes become unstable. So the term spans both the inherited disorders seen at birth and the chaotic genomes found in tumors.