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Aneuploidy: When the Count Is Off

Too many or too few chromosomes is called aneuploidy, and it almost always begins with one mishap during cell division: nondisjunction. Here is how trisomy and monosomy arise, and why they matter.

Euploid versus aneuploid

A cell with a balanced, complete set of chromosomes — 46 in humans — is said to be in euploidy. Aneuploidy is the opposite: a cell that has one or a few extra or missing chromosomes, so the total is not a clean multiple of the basic set. The two most common forms are trisomy (three copies of one chromosome instead of two, total 47) and monosomy (only one copy instead of two, total 45).

Aneuploidy is consequential because chromosomes are not interchangeable parts — each carries its own set of genes that the body expects in a precise dose. An extra chromosome means an extra copy of every gene on it; a missing one means half the usual copies. Cells are surprisingly sensitive to that imbalance, which is why most autosomal aneuploidies are not survivable and end very early in development. The ones that can be carried to birth — like trisomy 21 — tend to involve small, gene-poor chromosomes or the sex chromosomes, which the body buffers more gently.

The root cause: nondisjunction

To make eggs and sperm, the body runs meiosis — a special division that halves the chromosome number, so each gamete ends up with one copy of each chromosome (23 in total). The handoff depends on paired chromosomes pulling cleanly apart. When a pair fails to separate, the error is called nondisjunction: both copies travel into the same gamete, leaving the other gamete with none.

Normal meiosis (one chromosome pair shown):

   pair  ●●  --->  ●   and   ●        each gamete gets 1

Nondisjunction:

   pair  ●●  --->  ●●  and  (none)

   gamete with ●●  + normal partner ●  =  ●●●   TRISOMY (3 copies)
   gamete with none + normal partner ● =  ●     MONOSOMY (1 copy)

One slip in one parent's cell produces both outcomes,
depending on which gamete is fertilized.
Nondisjunction sends both chromosomes of a pair into one gamete. Fertilization then yields a trisomy or a monosomy.

If a gamete carrying two copies meets a normal partner carrying one, the embryo has three — a trisomy. If a gamete with zero copies is fertilized by a normal one, the embryo has only one — a monosomy. Nondisjunction can also happen after fertilization, during the embryo's own divisions, which produces a patchwork we will meet in the final guide.