Chromosomal Mutations and Aneuploidy

Aneuploidy means a cell has the wrong number of chromosomes, usually one too many or one too few. It happens when chromosomes fail to separate properly during meiosis, a mistake called nondisjunction. If a pair of homologous chromosomes (or two sister chromatids) stays stuck together instead of splitting, one gamete ends up with an extra chromosome and the other ends up missing one. Fertilize that gamete with a normal one, and the resulting zygote has 47 chromosomes instead of 46, or 45. An extra copy of a whole chromosome means every gene on it is now present in three copies instead of two. The genes themselves are fine, but their products are overproduced relative to everything else in the cell. Since so many proteins and regulatory networks depend on precise ratios, not just presence, this dosage imbalance is what causes developmental problems, not a broken gene.

Chromosomes can also break and get repaired incorrectly, producing four structural changes on a single chromosome. A deletion loses a segment entirely. A duplication copies a segment, raising the dosage of those genes the same way trisomy does, but only for part of the chromosome. An inversion flips a segment end to end, keeping all the genes but scrambling their order. A translocation moves a segment to a different, nonhomologous chromosome. Deletions and duplications change gene dosage; inversions and translocations usually don't, but they can disrupt genes at the break points or separate a gene from the regulatory sequences it needs.

These chromosomal mutations join point mutations as a random source of heritable variation. They don't arise to solve a problem or improve fitness; they happen by chance during cell division. Natural selection then acts afterward on whatever phenotypic differences result, keeping variants that help survival and reproduction and weeding out those that hurt it.