karyotype

A karyotype is like a group portrait of all of a cell's chromosomes, lined up and sorted so they can be inspected at a glance. By arranging the chromosomes from largest to smallest and pairing up the matching ones, biologists can see at once how many there are and whether any look unusual.

To make a karyotype, cells are captured at the stage of division when chromosomes are most condensed and visible, then stained to produce a pattern of light and dark bands. The chromosomes are photographed, cut out, and arranged in order: in humans, 22 numbered pairs of autosomes plus the sex chromosomes, written as 46,XX for a typical female or 46,XY for a typical male. The banding pattern lets specialists identify each chromosome and spot rearrangements.

Karyotyping is a cornerstone of cytogenetics. It reveals whole-chromosome changes such as having an extra copy (as in trisomy 21, the cause of Down syndrome) or a missing one, as well as large structural changes such as a piece broken off or swapped between chromosomes. It cannot, however, detect small changes within a gene — single-letter mutations are invisible at this scale and require DNA sequencing instead.