From a cell to a sorted portrait
A karyotype is a complete picture of all of a cell's chromosomes, photographed when they are most condensed and then arranged in a standard order. It is the foundational tool of cytogenetics — the study of chromosomes under the microscope — and it lets you literally count and inspect the set.
- Collect dividing cells and stop them at the stage where chromosomes are most condensed and visible.
- Swell and burst the cells so the chromosomes spread out without overlapping.
- Stain them so each chromosome shows a unique pattern of light and dark bands.
- Photograph the spread, then sort the chromosomes by size, centromere position, and banding.
- Pair up the homologs and number them 1 to 22, with the sex chromosomes placed last.
Autosomes, sex chromosomes, and reading the result
Of the 23 pairs, 22 are autosomes — the chromosomes numbered 1 to 22, which are the same in everyone. The 23rd pair are the sex chromosomes. A typical female karyotype has two X chromosomes (written 46,XX); a typical male has one X and one Y chromosome (46,XY). The X is large and gene-rich; the Y is small and carries far fewer genes.
Because every chromosome has an expected size, count, and banding pattern, a karyotype makes departures from the norm visible at a glance. An extra or missing whole chromosome — aneuploidy — shows up as the wrong number. The best-known example is trisomy 21, three copies of chromosome 21 instead of two, written 47,XX,+21 or 47,XY,+21. Reading a karyotype is descriptive, not a diagnosis on its own; it is one careful observation that doctors and geneticists interpret in context.
Reading the shorthand of a karyotype: 46,XX total 46 chromosomes; two X -> typical female set 46,XY total 46 chromosomes; X and Y -> typical male set 47,XY,+21 47 total; XY; one extra #21 -> trisomy 21 45,X only 45; a single X, no partner -> a missing sex chromosome Read it as: [ total count ] , [ sex chromosomes ] , [ what changed ]