Karyogram
The karyogram (karyotype) is the standardized map of the genome. Its construction is the final “quality control” step where the laboratory scientist verifies the identity of every chromosome. Errors in placement (e.g., swapping Ch 19 and 20) are considered major deficiencies in proficiency testing
Grouping & Ordering
Chromosomes are laid out in a specific order (1 through 22, then XY), typically in 3 or 4 rows
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Row 1 (Group A & B)
- 1, 2, 3: The largest
- 4, 5: Large sub-metacentrics
- Placement Error: Confusing 4 and 5 is common. Remember: 4 is lighter (light band 4q26); 5 is darker (dark band 5q12)
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Row 2 (Group C)
- 6, 7, 8, 9, 10, 11, 12: The hardest row
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Placement Error: Swapping 10 and 12 is the most frequent error
- 10: Three distinct bands on q-arm
- 12: Long q-arm, very dark centromere
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Placement Error: Swapping 8 and 9
- 8: Light, undefined
- 9: Darker, secondary constriction (9q12)
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Row 3 (Group D, E, F)
- 13, 14, 15 (D): Acrocentrics. 13 is lightest; 14 is intermediate; 15 is distinct (dark proximal, light distal)
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16, 17, 18 (E)
- 16: Metacentric (dark centromere)
- 17: Light p-arm
- 18: Dark overall
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19, 20 (F)
- 19: Light (GC rich)
- 20: Dark (AT rich)
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Row 4 (Group G & Sex)
- 21, 22: Acrocentrics. 21 is smaller. 22 is “boxier.”
- X, Y: X is placed next to or after Group C (size match) or at the end. Y is at the very end
Orientation Rules (ISCN)
Every chromosome must be oriented vertically with correct polarity
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The “p” is Up: The Short Arm (p) always faces the top of the page
- Metacentrics (1, 3, 16, 19, 20): Often hard to tell p from q. The p-arms are usually slightly shorter
- Sub-metacentrics (4, 5, 6-12, 17, 18, X): Easy. The short arm is obvious
- The “q” is Down: The Long Arm (q) faces the bottom
- Acrocentrics (13-15, 21-22, Y): The “Satellite” (stalk) is the p-arm. Therefore, the satellites must be pointing UP
- Centromere Alignment: The centromeres of all chromosomes in a row must form a straight horizontal line. This allows the reviewer to scan across and instantly see length discrepancies
Placement of Homologs & Abnormalities
- Standard Pair: Homologs (e.g., the two Ch 7s) are placed side-by-side
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Translocation
- If a translocation exists (e.g., t(11;14)), the layout changes to facilitate comparison
- Pair 11: Normal 11 (left) | Derivative 11 (right)
- Pair 14: Normal 14 (left) | Derivative 14 (right)
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Inversion
- Normal Homolog (left) | Inverted Homolog (right)
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Unidentified Marker
- Placed at the very end of the karyogram (after Y), labeled “mar”
Common Review Pitfalls
When reviewing a karyogram (e.g., for an exam or sign-out), look for these subtle errors:
- Inverted Polarity: Is Chromosome 2 upside down? (Checking the banding pattern of 2p vs 2q)
- Mislabeled C-Group: Did they put a Ch 9 in the Ch 10 spot? (Check for the secondary constriction)
- Missed Deletion: Is one Ch 5 shorter than the other? (del(5q) is subtle in condensed cells)
- Y Heterochromatin: Is the Y missing its dark distal end? (Deleted Yq)
- Satellite Stalks: Are there huge satellites on Ch 14? (14ps+ - a normal variant, but noteworthy)