Preparatory Mindset
Cytogenetics is about VISUALIZING chromosomes. Different techniques detect different-sized abnormalities — G-banding sees whole chromosome changes, FISH sees specific sequences, arrays see submicroscopic gains/losses. Match the technique to the clinical question.
Core Concepts
- G-banding: Giemsa staining after trypsin pretreatment → dark (AT-rich, gene-poor) and light (GC-rich, gene-rich) bands
- High-resolution banding: Arrest cells earlier in mitosis (prophase) → longer, less condensed chromosomes → more bands (800+)
- FISH: Fluorescent probes bind specific DNA sequences → detects translocations, aneuploidy, microdeletions
- CGH (Comparative Genome Hybridization): Test vs reference DNA → detects gains/losses (NOT balanced translocations)
- ISCN notation: Standard system for describing karyotypes: 46,XY = normal male; 47,XX,+21 = Down syndrome
- Metaphase arrest: Colcemid (colchicine analog) blocks spindle formation → arrests cells at metaphase for karyotyping
- Chromosome abnormalities: Numerical (aneuploidy, polyploidy) and Structural (deletions, duplications, translocations, inversions)
High-Yield Points
- Metaphase arrested with colcemid → best chromosome visualization
- G-banding: trypsin + Giemsa; dark = AT-rich; light = GC-rich
- FISH = targeted (probe for known sequence); Array CGH = genome-wide (unbalanced changes only)
- High resolution banding = earlier arrest (prophase), more bands
Topic Summary
Cytogenetic techniques range from G-banding (whole genome, low resolution) to FISH (targeted) to array CGH (genome-wide copy number). Each answers different clinical questions.