FISH
FISH, short for fluorescence in situ hybridization, is a technique that paints a specific DNA sequence with a glowing tag so you can see exactly where it sits on a chromosome. Think of it as a highlighter that finds one chosen sentence in the whole genome and makes it shine under the microscope. It bridges the gap between coarse karyotyping and fine DNA sequencing.
It works through base pairing. A short, single-stranded DNA probe is built to match the target sequence and is attached to a fluorescent dye. When applied to chromosomes on a slide, the probe binds its complementary region, and the spot lights up under a fluorescence microscope. The number and position of glowing dots reveal how many copies of that sequence are present and where.
FISH is especially powerful for things a karyotype cannot resolve: it can confirm a microdeletion by showing a missing signal, count gene copies, or reveal a translocation by tracking probes from two different chromosomes that have come together. It is widely used in prenatal testing and in cancer diagnostics, for instance to detect the BCR-ABL1 fusion of the Philadelphia chromosome.
Two probes flanking a translocation breakpoint, normally apart, light up next to each other in leukemia cells — FISH evidence of the Philadelphia chromosome.
Glowing probes turn an invisible rearrangement into a countable signal.
FISH only checks the specific spots its probes target — it can confirm or exclude a known change, but it will not scan the whole genome the way an array or sequencing can.