cytogenetics
Cytogenetics is the branch of genetics that looks at the cell's chromosomes the way a librarian inspects whole books rather than individual sentences. Instead of reading the exact spelling of a gene, it asks coarser but vital questions: How many chromosomes are there? Are any too long, too short, broken, or swapped? Because chromosomes are big enough to see under a light microscope when a cell divides, cytogenetics is one of the oldest ways to detect genetic abnormalities by eye.
The classic tool is the karyotype, an ordered photograph of a cell's chromosomes arranged by size and banding pattern. Cells are arrested in mitosis (when chromosomes are most condensed), stained to produce characteristic light and dark bands, and then sorted into 23 pairs in humans. Missing, extra, or rearranged chromosomes show up as departures from the expected 46-chromosome layout.
Modern cytogenetics blends this microscope tradition with molecular methods. Fluorescence in situ hybridization (FISH) tags specific DNA sequences with glowing probes, and chromosomal microarrays scan the genome for missing or duplicated chunks too small for the eye. Together these reveal aneuploidies, translocations, deletions, and duplications that underlie many congenital syndromes and cancers.
A laboratory grows a patient's white blood cells, halts them in mitosis, stains them, and counts 47 chromosomes with three copies of chromosome 21 — a cytogenetic diagnosis of Down syndrome.
Counting and arranging chromosomes turns an abnormality into a visible diagnosis.
Cytogenetics sees big structural changes; it cannot read a single misspelled base. A normal karyotype does not rule out a single-gene disorder, which needs DNA sequencing instead.