G-Banding Patterns: How to Identify Chromosomes 1 to 22, X and Y
Last updated: October 22, 2026
Every human chromosome has a G-banding pattern of light and dark bands that identifies it, as long as you know which landmarks to look for. This reference covers the chromosome groups, the landmark bands of each chromosome, and the pairs trainees confuse most often.
How G-banding works
G-banding treats metaphase chromosomes with trypsin and then stains them with Giemsa. The result is a pattern of dark and light bands that is the same in every normal copy of a given chromosome. Dark G-bands are generally AT-rich, gene-poor and late-replicating. Light bands are GC-rich and gene-rich.
Bands are named by chromosome, arm, region and band. In 1q12, 1 is the chromosome, q is the long arm, 1 is the region and 2 is the band. You read it "one q one two", not "one q twelve". Sub-bands follow a decimal point, as in 22q11.2.
The landmarks below describe chromosomes at the band levels used in routine analysis, about 400 to 550 bands. At higher resolution many bands split into sub-bands, but the overall pattern stays recognizable.
Start with size, centromere and group
Before you look at bands, sort each chromosome by its size and centromere position. A metacentric chromosome has its centromere near the middle, a submetacentric one has it off-center, and an acrocentric one has it near one end, with a tiny short arm that often carries satellites.
| Group | Chromosomes | Size and shape |
|---|---|---|
| A | 1, 2, 3 | Largest. 1 and 3 metacentric, 2 submetacentric. |
| B | 4, 5 | Large, submetacentric, short p arms. |
| C | 6 to 12, X | Medium, submetacentric. |
| D | 13, 14, 15 | Medium acrocentrics with satellites. |
| E | 16, 17, 18 | Small. 16 nearly metacentric, 17 and 18 submetacentric. |
| F | 19, 20 | Small metacentrics. |
| G | 21, 22, Y | Smallest. 21 and 22 acrocentric with satellites, Y without. |
Chromosomes 1 to 5 (groups A and B)
| Chromosome | Landmarks to look for | Often confused with |
|---|---|---|
| 1 | The largest chromosome, metacentric. A block of heterochromatin just below the centromere on the long arm (1q12), whose size varies between people. The distal half of the short arm is pale, with fine bands. The long arm has several well-spaced dark bands. | 2, when 1 is bent or poorly spread |
| 2 | The largest submetacentric. Both arms carry many evenly spaced dark bands, about four of them prominent on the short arm. No heterochromatin block near the centromere. | 1 |
| 3 | Large and metacentric, with a nearly symmetrical pattern: dark bands on each side of the centromere, a pale zone in the middle of each arm (3p21 and 3q21), then darker bands toward both ends. Often compared to a bow tie. | Rarely confused once the symmetry is seen |
| 4 | Large submetacentric with a short p arm. The long arm is evenly banded, with dark bands of similar intensity along its whole length. | 5 |
| 5 | Same size and shape as 4, but the long arm is blockier: a dark middle section (5q14 to 5q23), a pale region distal to it, a dark band group near the end, and a pale tip. The short arm has one prominent dark band in its middle (5p14). | 4 |
4 versus 5: the long arm of 4 looks uniformly striped from top to bottom. The long arm of 5 has a dark central block followed by a clearly paler stretch.
Chromosomes 6 to 12 and X (group C)
The C group is where most identification errors happen. Work from the distinctive chromosomes (6, 7, 9 and X) to the less distinctive ones (8, 10, 11 and 12).
| Chromosome | Landmarks to look for | Often confused with |
|---|---|---|
| 6 | The largest C-group chromosome. A distinct pale band in the middle of the short arm (6p21), often called the "window". The long arm has several evenly spaced dark bands. | 7, X |
| 7 | A dark band at the very top of the short arm (7p21), like a dark cap. The long arm has two prominent dark bands (7q21 and 7q31) separated by a pale band. | X, 6 |
| 8 | The least distinctive C-group chromosome, with fairly even banding. The short arm has two dark bands separated by a pale band, and the long arm has evenly spaced dark bands. It is often identified by elimination. | 10 |
| 9 | A pale heterochromatin region just below the centromere on the long arm (9q12) gives a pale "neck". The distal long arm has several dark bands. A pericentric inversion of this region, inv(9)(p12q13), is a common normal variant. | 10, 11 in poor spreads |
| 10 | The long arm has three clear dark bands, evenly spaced, the one nearest the centromere (10q21) being the most prominent. The short arm is fairly uniformly grey. | 8, 9 |
| 11 | A longer short arm than 12. On the long arm, a pale region next to the centromere (11q13) is followed by a large dark block farther out (11q14 to 11q22). | 12 |
| 12 | A short p arm with a single dark band. On the long arm, the large dark block sits close to the centromere (12q14 to 12q21), after only a short pale band. | 11 |
| X | Between 7 and 8 in size. One prominent dark band in the middle of the short arm (Xp21) and another at a matching distance on the long arm (Xq21), which gives an almost mirror-image look. | 7 |
11 versus 12: on chromosome 12 the dark block on the long arm starts close to the centromere and the short arm is shorter. On chromosome 11 there is a longer pale stretch before the dark block.
Chromosomes 13 to 15 (group D)
| Chromosome | Landmarks to look for | Often confused with |
|---|---|---|
| 13 | The largest acrocentric. The long arm gets darker toward its end: the darkest bands (13q21 and 13q31) lie in the distal half, and the part near the centromere is paler. | 14, 15 |
| 14 | Dark bands near the centromere and near the end of the long arm (around 14q21 and 14q31), with a paler region between them. | 13, 15 |
| 15 | The darkest staining is in the proximal half of the long arm, and the distal half is paler. This is roughly the reverse of 13. | 14 |
The short arms and satellites of all three vary from person to person and are not used for identification.
Chromosomes 16 to 20 (groups E and F)
| Chromosome | Landmarks to look for | Often confused with |
|---|---|---|
| 16 | Nearly metacentric, with a block of heterochromatin just below the centromere on the long arm (16q11.2) that varies in size. Each arm carries a couple of dark bands. | Rarely confused in good spreads |
| 17 | Submetacentric. The long arm has a pale region near the centromere (17q21), one dark band farther out (17q22 to 17q24), and a pale tip. | 18 |
| 18 | Submetacentric with a very short p arm. The long arm is mostly dark, with dark bands both near the centromere and in its distal part. | 17 |
| 19 | Small and metacentric, mostly pale, with dark staining around the centromere. | 20 |
| 20 | Small and metacentric. The short arm stains darker than the long arm, which is mostly pale. | 19 |
Chromosomes 21, 22 and Y (group G)
| Chromosome | Landmarks to look for | Often confused with |
|---|---|---|
| 21 | The smallest autosome. Acrocentric, with a large dark band on the long arm close to the centromere (21q21) that makes the arm look dark. | 22 |
| 22 | Acrocentric and slightly longer than 21, but mostly pale, with only a small dark band near the centromere (22q12). | 21 |
| Y | G-group size, with no satellites. The heterochromatin at the end of the long arm (Yq12) varies greatly in length between men and often looks darker or fuzzy. The chromatids of the long arm often lie parallel. | 21, 22 |
21 versus 22: the numbering is historical, and 21 is actually the smaller chromosome. Look at the long arm: 21 is dark close to the centromere, while 22 is pale apart from a small dark band.
Tips for faster identification
- Find the easy ones first: 1, 3, 9, the X and the G group. Then sort the rest by elimination.
- Count each group: a normal female has 16 C-group chromosomes (seven pairs plus two X chromosomes) and a normal male has 15. An unexpected count points to a sex chromosome change or a misidentified chromosome.
- Compare homologs with each other, not only with an ideogram. Two copies of the same chromosome in the same cell are the best reference you have.
- Expect variation: heterochromatin blocks, satellites and the Yq length differ between people and are not abnormalities.
- Practice on many cells. Recognition speed comes from volume, not from memorizing diagrams.
Keep learning
This page is for education and training. It is not medical advice and must not be used to diagnose or treat anyone. Laboratory results should be interpreted by qualified professionals following current standards.