What cytogenetics looks at
Most of genetics zooms in to the level of single letters of DNA. Cytogenetics zooms back out. It studies whole chromosomes — the long, packaged threads of DNA you can actually see under a microscope when a cell divides. A human body cell is diploid: it carries two copies of each chromosome, one inherited from each parent, for 46 chromosomes in total. Twenty-two of those pairs are autosomes (numbered 1 to 22 by size), and one pair is the sex chromosomes.
A karyotype is the standard way to lay this out. Technicians arrest cells in the middle of division (when chromosomes are most condensed and visible), stain them, photograph them, and then digitally sort the chromosomes into ordered pairs from largest to smallest. The finished karyotype is written in shorthand: a typical female is 46,XX and a typical male is 46,XY — the number of chromosomes, then the sex chromosomes present.
Bands, arms, and addresses
Each chromosome has a pinched waist called the centromere. It divides the chromosome into a short arm (labelled p, for petit) and a long arm (q). Stains produce a reproducible pattern of light and dark stripes called bands, and these bands are numbered outward from the centromere. Together they give every spot on every chromosome an address — for example, 17q21 means chromosome 17, long arm, region 2, band 1. That addressing system is how cytogeneticists describe exactly where a piece is missing or extra.
Karyotype shorthand, read left to right: 46 , XX | | | sex chromosomes present (XX = typical female) total chromosome count 46 , XY typical male 47 , XX , +21 a female with one extra chromosome 21 45 , X a single X, no second sex chromosome The "+" means an extra copy; "-" means a missing one. Numbers after the sex chromosomes flag which chromosome is involved.
Why does this picture matter so much? Because a wrong number of chromosomes, or a chromosome with a chunk added, lost, or moved, changes the dose of hundreds or thousands of genes at once. The rest of this track is about exactly those changes — and the karyotype, with about 400–550 visible bands, is the first tool for spotting them.