Revision summary
Karyotype means chromosome complement; karyogram is its ordered image. Metaphase arrest, hypotonic spreading and G-banding permit homologous pairing and notation. It diagnoses trisomy 21, Turner, Klinefelter and other numerical aberrations. It also finds large deletions, inversions and balanced or unbalanced translocations. Multiple cells must be examined for mosaicism. A normal karyotype does not exclude microdeletions or single-gene disease; use FISH, microarray or sequencing appropriately.
Model answer
Introduction
A karyotype is an individual’s chromosome complement described by number, size, centromere position and banding pattern; a karyogram is its ordered visual display. Conventional analysis examines condensed metaphase chromosomes and is especially useful for whole-chromosome changes and large structural rearrangements.
Body
Preparation and reading
Dividing cells from blood, amniotic fluid, chorionic villi, marrow or tumour are cultured where necessary. Colchicine/colcemid arrests metaphase; hypotonic treatment spreads chromosomes; fixation, staining and microscopy follow. G-banding produces reproducible light and dark bands. Homologues are paired from chromosome 1 to 22 plus sex chromosomes, producing notations such as 46,XX or 46,XY.
Analysis checks:
- total chromosome number;
- homologous size and centromere position;
- missing, duplicated or rearranged bands;
- several cells to identify mosaicism.
Diagnostic value
- Numerical aberrations: meiotic nondisjunction may produce trisomy 21 (47,XX,+21), trisomy 18, trisomy 13, Turner syndrome (45,X) or Klinefelter syndrome (47,XXY). Polyploid conceptions can also be recognised.
- Structural aberrations: large deletions such as 5p deletion in cri-du-chat syndrome, duplications, inversions, ring chromosomes and reciprocal or Robertsonian translocations are visible when above the banding resolution.
- Reproductive diagnosis: A phenotypically normal parent may carry a balanced translocation. Karyotyping can explain recurrent miscarriage or a child with an unbalanced rearrangement and guide recurrence-risk counselling.
- Cancer cytogenetics: acquired rearrangements, classically the Philadelphia chromosome t(9;22) in chronic myeloid leukaemia, can classify disease.
Targeted FISH detects specified loci and rearrangements; chromosomal microarray detects smaller copy-number changes; sequencing detects nucleotide variants. These complement rather than make every karyotype obsolete.
Limits
Conventional resolution is usually several megabases and depends on band quality. It misses most single-gene mutations, small copy-number variants and many low-level mosaics. A normal karyotype therefore does not mean a normal genome, while a prenatal abnormality requires careful, non-directive interpretation rather than deterministic prediction.
Flow diagram
flowchart LR C[Dividing cells] --> M[Metaphase arrest] M --> S[Spread stain band] S --> K[Ordered karyogram] K --> N[Numerical change] K --> R[Large rearrangement] K --> F[FISH microarray sequencing if needed]
Conclusion
Karyotyping converts chromosome morphology into a diagnosis of aneuploidy and major rearrangement. Its strength is the genome-wide view of large changes; its limit is resolution. Sound diagnosis combines clinical findings, adequate cell sampling and the appropriate molecular follow-up.
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Why retain karyotyping after microarray?
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