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A karyotype is a visual representation of an individual's chromosomes arranged by size, centromere position, and banding pattern. G-banding (Giemsa banding) is the standard technique, providing a genome-wide view of chromosome number and large structural rearrangements at a resolution of approximately 5–10 Mb.
- G-banding: Chromosomes are treated with trypsin and stained with Giemsa dye, producing a characteristic light and dark banding pattern. Each chromosome has a unique banding signature that allows identification.
- Resolution: Typically 400–550 bands per haploid set at routine analysis; high-resolution banding (prometaphase) can achieve ~850 bands, improving detection of smaller rearrangements.
- Cell culture required: Peripheral blood lymphocytes are stimulated with phytohemagglutinin (PHA) and cultured for ~72 hours. Other tissue types (amniocytes, fibroblasts, bone marrow) require different culture conditions and times.
- Metaphase arrest: Colcemid is added to arrest cells in metaphase, when chromosomes are maximally condensed and visible.
- Minimum cell count: Typically 20 metaphases analyzed to rule out mosaicism at ~14% level (95% confidence). More cells needed if mosaicism is suspected.
- Turnaround time: 7–14 days for peripheral blood; 10–21 days for amniotic fluid; 1–3 days for bone marrow (direct or short-term culture).
- Numerical abnormalities: Aneuploidy (trisomy, monosomy), polyploidy (triploidy, tetraploidy)
- Large structural rearrangements: Translocations (balanced and unbalanced), large deletions/duplications, inversions, ring chromosomes, isochromosomes, marker chromosomes
- Balanced rearrangements: A key advantage over CMA, since karyotype can detect balanced translocations and inversions
- Mosaicism: Can be detected by analyzing multiple cells individually
- Low resolution: Cannot detect CNVs smaller than ~5–10 Mb
- Requires dividing cells: Cannot be performed on non-viable tissue or samples that fail to grow in culture
- Culture artifacts: Pseudomosaicism can arise from in vitro culture
- Cannot detect: Point mutations, small deletions/duplications, methylation abnormalities, uniparental disomy
- Labor-intensive: Manual analysis by trained cytogeneticists; lower throughput than molecular methods
- Suspected aneuploidy (e.g., Down syndrome, Turner syndrome)
- Recurrent pregnancy loss (to identify balanced translocations in parents)
- Ambiguous genitalia or suspected sex chromosome abnormality
- Hematologic malignancies (bone marrow karyotype)
- When a balanced rearrangement is suspected (CMA will miss these)
- Prenatal diagnosis via amniocentesis or CVS
"Karyotype Catches Balanced": unlike CMA, karyotype can detect balanced translocations and inversions because it visualizes chromosome structure directly.
"5 to 10 Mb is the Limit": karyotype resolution is approximately 5–10 Mb. Anything smaller requires CMA, FISH, or molecular testing.