Chromosome Shape Classification: Metacentric, Submetacentric, and Acrocentric

Published 2026-09-07

Before you can read bands you need to read shape. Where the centromere sits decides how a chromosome looks in a karyogram, which group it files under, and which neighbours it can be confused with. This guide walks through the three human classes, the one class humans do not have, and an interactive classifier you can scroll through.

In brief. Chromosome shape is simply where the centromere sits.

• Metacentric: the two arms are about the same length: 1, 3, 16, 19, 20.
• Submetacentric: there is a clear short arm and a clear long arm: 2, 4, 5, 6, 7, 8, 9, 10, 11, 12, 17, 18, X.
• Acrocentric: the centromere is near one end, so the chromosome is mostly q arm: 13, 14, 15, 21, 22, Y. Chromosomes 13, 14, 15, 21 and 22 carry satellites; Y does not.
• Telocentric: only one arm. Normal humans do not have these; mice do.

You can describe the same idea with the centromeric index (p arm ÷ total length) and the arm ratio (q ÷ p). Those formulas are below for completeness, but do not memorize the cut-offs. If you can look at a chromosome in the tool below and the class looks right, you already have the gist.

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Start at the centromere

Every chromosome you sort in a karyogram has one primary constriction: the centromere, the waist where the two sister chromatids are held together and where the spindle attaches at mitosis. It divides the chromosome into two arms. The shorter arm is the p arm (from the French petit), and the longer arm is the q arm, simply the next letter in the alphabet. By convention the p arm is drawn on top, so a karyogram is a row of chromosomes standing with their short arms up.

That single landmark carries a lot of information. Two chromosomes of the same length with the centromere in different places look completely different on the slide, and in the early days of cytogenetics, before banding, centromere position and length were the only things anyone had to work with. They are still the first two things an experienced tech checks before looking at a single band.

Two numbers that describe shape

Shape is measured, not eyeballed, when it matters. Two ratios are in common use and they say the same thing in different ways.

Centromeric index (CI) = p arm length ÷ total length
Arm ratio (r)          = q arm length ÷ p arm length

A centromere dead centre gives a CI of 0.50 and an arm ratio of 1.0. As the centromere slides toward one end the CI falls toward zero and the arm ratio climbs. Levan, Fredga and Sandberg proposed the thresholds most textbooks still quote in 1964, and they are the ones the classifier uses in its free-explore mode.

Real chromosomes do not always sit politely inside those boxes. Chromosome 16 is a good example: by strict measurement it sits at the boundary, and different references file it as metacentric or submetacentric. The human grouping used in the tool follows the conventional karyotype groups A through G, which is what you will be asked to know for the ASCP CG exam.

Metacentric: arms about equal

A metacentric chromosome has its centromere close to the middle. In a spread it looks like a symmetrical X; in a karyogram, standing up, the two arms are near mirror images in length. The banding is what separates them from one another. Chromosome 1 is the textbook case: the largest chromosome in the set, with a pale block of heterochromatin just under the centromere on the q arm (1q12) that is easy to spot. Chromosome 3 is the other large metacentric. Chromosomes 19 and 20 are the small metacentrics of group F, and telling those two apart is mostly about tone: 19 is pale on both arms, 20 is darker with visible bands on q.

Submetacentric: a clear short arm and a clear long arm

Most human chromosomes are submetacentric. The centromere is plainly off centre, so there is a definite short arm and a definite long arm, but the short arm is still a real arm with bands of its own. Chromosome 2 is almost as long as 1 and is the easiest way to see the difference between the two classes side by side: similar length, very different centromere. The group B pair, 4 and 5, are long submetacentrics with short pale p arms and long banded q arms, and they are confused with each other more than with anything else. The group C chromosomes, 6 through 12 plus the X, are the medium submetacentrics where banding does almost all the work, and where landmarks earn their keep: the pale window on 6p, the bright cap on 7p, the pale block under the centromere on 9 that makes it look like a figure in a dress, the single band across 10p.

Tip for trainees. The X files with group C by size and shape and is regularly mistaken for a 9. Both look like a figure in a dress. The X is darker overall with clear black and white banding and a distinct dark band in the middle of the q arm.

Acrocentric: the centromere near one end

In an acrocentric chromosome the centromere sits close to the tip, so the p arm is tiny. In humans that tiny p arm is special. On 13, 14, 15, 21 and 22 it carries a short stalk and a small knob called a satellite. The stalks are the nucleolar organiser regions, where the ribosomal RNA genes live, and the satellites are the little chromatin blobs beyond them. They are variable, they sometimes appear to touch between chromosomes in a spread (satellite association), and they matter clinically: the acrocentrics are the chromosomes involved in Robertsonian translocations, where two of them fuse at the centromere and the short arms are lost without consequence because the rRNA genes are redundant.

The Y is the sixth acrocentric. It has no satellites, and most of its q arm is heterochromatin that stains dark and varies in length between men, which is normal. Trainees confuse a Y with a 21 or 22 until they check for satellites and look at how dark the bottom of the q arm is.

Telocentric: the class humans do not have

A telocentric chromosome has its centromere at the very end, so it has a single arm. It is a real category, and a normal one in some species: every mouse chromosome is telocentric, which is why a mouse karyogram looks like rows of single rods rather than X shapes. But no normal human chromosome is telocentric. Even the acrocentrics keep a short p arm, and the true terminal structure of a human chromosome is a telomere, not a centromere.

If a human chromosome looks telocentric, something has happened to it or to the image. The usual explanations are a terminal deletion that removed the short arm, an isochromosome made of two copies of one arm, or simply a chromosome folded over in the spread so its short arm is hidden. Rotate it, check the metaphase, and count the arms before you classify it.

Using shape when you karyotype

In practice, shape narrows the field before banding finishes the job. A workable routine for a new tech:

1. Stand the chromosome up with the short arm on top. If you cannot tell which arm is shorter, it is probably metacentric.
2. Place it by size against the ones already sorted. Length alone separates the A group from the F and G groups.
3. Check the centromere. Middle means 1, 3, 16, 19 or 20. Off centre means group B, C or E. Near the tip with satellites means D or G; near the tip without satellites and dark below means Y.
4. Only now read the bands, starting with the landmark for that candidate: the 1q12 block, the 6p window, the 9q12 constriction, the 16q11 block, the dark line under the centromere on 21.

The classifier above is built for exactly this drill. Scroll through the 24 human chromosomes, drawn to scale from the ISCN 550-band ideograms, and watch the p arm, q arm, centromeric index and class update for each one. Switch to the arm-ratio view to walk through all 24 sorted by centromeric index, most metacentric first and stretched to one length, so only the arm ratio changes as you scroll. Switch to free explore to drag a centromere from the middle to the tip and see where the class boundaries fall, including the telocentric zone humans never reach. In free explore, every class lists the human chromosomes that belong to it, and clicking a number jumps straight to that chromosome.

Common questions

What decides whether a chromosome is metacentric, submetacentric or acrocentric? The position of the centromere. Measured as the centromeric index (short arm length divided by total length), a centromere near the middle (about 0.37 to 0.50) makes a metacentric chromosome, one clearly off centre (about 0.25 to 0.37) makes a submetacentric chromosome, and one near the end (about 0.12 to 0.25) makes an acrocentric chromosome.

Which human chromosomes are metacentric? By convention chromosomes 1, 3, 16, 19 and 20. Their two arms are close to equal in length, so they look symmetrical in a karyogram.

Which human chromosomes are acrocentric? Chromosomes 13, 14, 15, 21 and 22 are acrocentric with satellites on their short arms, and the Y chromosome is also acrocentric but has no satellites. All the other autosomes and the X are submetacentric.

Do humans have telocentric chromosomes? No. A telocentric chromosome has its centromere at the very tip and only one arm. Every normal human chromosome keeps at least a short p arm. Mouse chromosomes are all telocentric. A human chromosome that appears telocentric usually has a terminal deletion, is an isochromosome, or is folded in the metaphase spread.

What is the centromeric index? The length of the short (p) arm divided by the total length of the chromosome, so it runs from 0.5 for a perfectly metacentric chromosome down toward 0 for a telocentric one. The arm ratio is the related measure, long arm divided by short arm.

Why does chromosome shape matter when karyotyping? Shape narrows the candidates before you read the bands. Together with size it places a chromosome into groups A to G, so a technologist only has to compare banding patterns among a few look-alikes instead of all 24.

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CytoSnap is also available if you want to cut and sort your own metaphase; Case 1 is free for everyone.

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