Karyotyping and FISH

USMLE Step 1 trap: Overestimates karyotype resolution, believing it can detect microdeletions detectable only by FISH. Standard karyotyping detects only large chromosomal abnormalities (>5–10 Mb); microdeletions like 22q11.2 require FISH or chromosomal microarray for detection.

Karyotyping and FISH are two cytogenetic techniques that show up on USMLE Step 1 primarily in genetics, oncology, and prenatal diagnosis questions. Karyotyping gives you a visual map of all 46 chromosomes by arresting cells in metaphase and staining them — it's great for catching big structural problems like trisomies, large deletions, or translocations that shift large chunks of chromosome. FISH uses fluorescent probes that hybridize to a specific DNA sequence, letting you ask a targeted question: 'Is this particular locus present, deleted, or rearranged?' The two techniques are complementary, and the exam loves to test whether you know which one to reach for in a given clinical scenario.

The trickiest part is knowing the resolution limits of each technique. Students consistently overestimate what karyotyping can see — if a question describes DiGeorge syndrome or another microdeletion syndrome, karyotyping will look normal. You need FISH or chromosomal microarray to catch a 22q11.2 deletion. On the flip side, students underestimate FISH, thinking it only finds deletions. FISH is also the go-to for confirming specific translocations in cancer — BCR-ABL in CML and t(15;17) in APL are classic USMLE Step 1 targets. A question might describe a patient with chronic myelogenous leukemia and ask which technique confirms the Philadelphia chromosome at the molecular level.

One underappreciated detail is why prenatal samples require cell culture before karyotyping. Amniocytes and chorionic villi cells need to be grown in culture first because karyotyping demands actively dividing cells that can be arrested in metaphase — you can't do it on a static tissue sample. FISH, by contrast, can be performed on non-dividing interphase cells, which is part of why it's faster and more flexible for urgent clinical questions.

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Common misconceptions

Common mistake

Wrong: Karyotyping can detect small microdeletions such as 22q11.2 deletion.

Right: Standard karyotyping detects only large chromosomal abnormalities (>5–10 Mb); microdeletions like 22q11.2 require FISH or chromosomal microarray for detection.

Standard karyotyping can only resolve chromosomal changes larger than roughly 5–10 megabases — anything smaller is invisible at light microscopy, even with expert banding. The 22q11.2 deletion (DiGeorge/velocardiofacial syndrome) involves only about 1.5–3 Mb, so the karyotype looks completely normal. If you see a clinical picture consistent with a microdeletion syndrome, karyotyping is the wrong answer — FISH with a probe targeting that specific locus, or chromosomal microarray, is what actually makes the diagnosis.

Common mistake

Wrong: FISH can only detect deletions, not translocations.

Right: FISH uses fluorescent probes to detect both microdeletions and specific translocations (e.g., BCR-ABL in CML, t(15;17) in APL).

FISH isn't limited to finding missing sequences — it can detect translocations by using dual-color probes that flank a breakpoint or label each translocation partner a different color. When the translocation occurs, the two colors appear fused or rearranged rather than in their normal positions. This is exactly how BCR-ABL fusion (Philadelphia chromosome in CML) and the PML-RARA fusion in APL are confirmed at the molecular cytogenetic level on peripheral blood or bone marrow samples.

Common mistake

Gap: Missing that karyotyping requires metaphase-arrested dividing cells, explaining why culture is needed for prenatal samples

Karyotyping requires actively dividing cells arrested in metaphase, which is why amniocytes or chorionic villi must be cultured before analysis.

Karyotyping requires chromosomes to be condensed and visible, which only happens during cell division — specifically metaphase. Amniocytes from amniocentesis and chorionic villus cells aren't naturally dividing fast enough in the sample, so they must be cultured for days to weeks to produce enough dividing cells, which are then arrested in metaphase with colchicine. FISH sidesteps this entirely because fluorescent probes can hybridize to the DNA in non-dividing interphase nuclei, making FISH faster and suitable for urgent prenatal or oncologic testing.

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What the exam tests

Know what karyotyping is, how it works (metaphase arrest, chromosome banding), and which categories of chromosomal abnormalities it can and cannot detect — including when it would return a normal result despite a real genetic disorder being present.
Know how FISH works mechanistically (fluorescent probe hybridization to a specific locus) and recognize the clinical scenarios where it's the right tool: confirming microdeletions like 22q11.2 and detecting specific oncogenic translocations like BCR-ABL (CML) and t(15;17) (APL).

Can you avoid these mistakes?

A newborn has conotruncal cardiac defects, hypocalcemia, and T-cell deficiency. The karyotype returns 46,XX — normal. What is the next best test to confirm the suspected diagnosis, and why did karyotyping miss it?

A 45-year-old is diagnosed with CML. The hematologist wants to confirm the BCR-ABL fusion at the chromosomal level and monitor for residual disease. Which technique is most appropriate and why — karyotype, FISH, or both?

Why must amniocytes collected via amniocentesis be cultured before karyotyping can be performed, and which technique could be applied directly to non-cultured amniocytes if a rapid result is needed?

A question stem describes a patient with a syndrome caused by a 1.5 Mb deletion on chromosome 15q11-13 (Angelman or Prader-Willi). A classmate argues that karyotyping would show a visible deletion on chromosome 15. Are they right? What resolution limit is relevant here, and what test should be ordered?

Karyotyping and FISH

Common misconceptions

What the exam tests

Can you avoid these mistakes?

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