How to Read a Karyotype: Chromosomes Explained

What You’ll Need

A clear karyotype image or laboratory report, preferably with the full written result and any explanatory notes.
A way to zoom in on the image. Printed copies can be difficult to inspect if chromosome labels are small.
Basic familiarity with chromosome numbers and notation, described below. You do not need to identify every chromosome by sight to understand the main parts of a report.
If the result is your own or concerns a family member, access to the clinician or genetic counselor who ordered the test. A karyotype is a laboratory finding, not a complete diagnosis by itself.

Step-by-Step

Check what kind of result you have. A karyotype image shows chromosomes photographed and arranged for comparison. A written result summarizes what the laboratory observed, often in standardized notation. Confirm whose sample it is and whether the report describes a blood, prenatal, bone marrow, or other sample; the context affects what was examined and what the result can answer.
Find the chromosome count. In a typical human karyotype, the displayed total is 46 chromosomes, arranged as 23 pairs. The first 22 pairs are autosomes; the final pair is the sex chromosomes, commonly XX or XY. A different count can be important, but first check whether the image is complete and whether the report notes a special cell line or test limitation.
Locate the pairs in order. Chromosomes 1 through 22 are generally arranged from larger to smaller, using size, banding patterns, and centromere position to distinguish them. The sex chromosomes are shown at the end. Pair members should look broadly alike, though they are not necessarily identical in every visible detail.
Look for the sex chromosome notation. In a written result, the number usually comes first, followed by the sex chromosome complement, such as 46,XX or 46,XY. These are common karyotype descriptions, not a complete account of sex development or gender. Some results have other complements or variations, which require individualized interpretation.
Read the full notation from left to right. The number before the comma is the chromosome count; the letters and symbols after it describe the sex chromosomes and any reported finding. For example, 47,XX,+21 describes a count of 47, an XX complement, and an additional chromosome 21. This is an example of notation, not a diagnosis of a particular person’s overall health.
Identify any reported change. A plus sign can indicate extra chromosome material or an additional chromosome, while a minus sign can indicate missing material or a missing chromosome. Letters such as “del” (deletion), “dup” (duplication), “t” (translocation), and “inv” (inversion) describe types of structural rearrangement. The exact meaning depends on the full notation and the laboratory’s findings.
Check for mosaicism or cell details. A result may list more than one cell line, separated by a slash, such as 46,XX/47,XX,+21. This indicates that the laboratory observed different chromosome patterns among the cells examined. It does not, by itself, tell you how every tissue in the body is affected or predict an individual’s traits.
Read the laboratory comment and limits. Check how many cells were analyzed, whether the result was reported as normal or abnormal, and whether the lab recommends additional testing. A conventional karyotype detects many chromosome number changes and larger structural changes, but it may miss small DNA changes or changes present in a small fraction of cells.
Discuss the result with the ordering professional. Ask what the finding means for the reason the test was ordered, whether it is conclusive, and whether follow-up testing or family testing is relevant. If the notation is unclear, request a plain-language explanation rather than trying to infer a diagnosis from the image alone.

Common Mistakes

Assuming 46 means everything is normal. The count can be typical while a structural rearrangement is present. The complete report matters, not just the total.
Treating the image as a diagnosis. A karyotype describes chromosome findings in examined cells. It does not on its own establish every health outcome, explain all symptoms, or predict a person’s future.
Confusing a karyotype with other genetic tests. A chromosome microarray, gene panel, or DNA sequencing test answers different questions and may detect changes a karyotype cannot. A normal karyotype does not rule out every genetic condition.
Overreading one cell or an unclear image. Image quality, preparation, and the number of cells analyzed matter. Laboratory reports account for technical criteria that may not be apparent from a screenshot.
Interpreting XX or XY as a complete description of identity or development. These labels report sex chromosome findings in the tested cells; they do not determine gender identity and may not describe every aspect of sex development.

Tips

Start with the written interpretation, then use the image to see how chromosomes are grouped. The report is generally more reliable than visual inspection of a low-resolution copy.
Keep the exact notation, including commas, plus or minus signs, parentheses, and any slash between cell lines. Small differences can change the meaning.
Ask for the full report if you were given only a brief summary. The laboratory comment may explain uncertainty, sample limitations, or why a finding was not classified as a diagnosis.
Write down questions before a genetics appointment: What did the test detect? What could it miss? Does the result affect relatives? Is another test needed to answer the original question?
Do not compare two reports without considering the sample, test method, and reason for testing. Different methods can produce results that are not directly interchangeable.

Troubleshooting

The count seems wrong: Make sure you are viewing the complete image and counting chromosomes rather than pairs. Some laboratory images include extra cells, labels, or partial chromosome spreads; rely on the reported result if the display is cropped or unclear.

The notation has unfamiliar symbols: Copy it exactly and ask the ordering clinician or laboratory to explain each part. Do not guess at abbreviations or assume a symbol has the same meaning outside standard chromosome notation.

The report says “variant,” “uncertain,” or gives a qualified result: Ask what was observed, how certain the laboratory is, and whether the finding is related to the reason for testing. A qualified finding should not be treated as either a confirmed diagnosis or proof that nothing is present.

The image and summary appear to disagree: Contact the laboratory or ordering clinician before drawing conclusions. The image may show selected cells, while the report reflects the full analysis and technical review.

You expected an answer but the result is normal: Ask what conditions the test can and cannot detect and whether another test is appropriate for the question. A normal karyotype means no reportable change was identified within the test’s scope; it does not rule out all genetic differences.

Frequently Asked Questions

What is a normal human karyotype?

A common description is 46,XX or 46,XY: 46 chromosomes in total, including two sex chromosomes. Other chromosome patterns exist, and whether a result is clinically significant depends on the specific finding and context.

Can I read a karyotype from a picture?

You can learn the arrangement and recognize the broad parts, but a picture alone may not show subtle changes or laboratory qualifications. Use the written report and ask a qualified professional to interpret an individual result.

What does 47,XX,+21 mean?

It describes 47 chromosomes, an XX sex chromosome complement, and an additional chromosome 21. The notation explains the chromosome finding; a clinician or genetic counselor can discuss its meaning in the specific testing context.

Does a normal karyotype rule out a genetic condition?

No. Karyotyping can miss small DNA changes, some low-level mosaicism, and changes outside the resolution of the method. Other tests may be needed depending on the clinical question.

What does mosaicism mean on a karyotype?

It means the laboratory found more than one chromosome pattern among the cells examined. The proportions in the sample do not necessarily represent every tissue, so interpretation depends on the sample and reason for testing.

Further reading: MedlinePlus Genetics. Details can change, so check the official page for anything that matters to you. Spotted a mistake? Tell us. How we write an answer.