A karyotype is the complete set of chromosomes inside your cells, and also the organized image scientists create by photographing, cutting out, and arranging those chromosomes by size and shape. A normal human karyotype contains 46 chromosomes: 22 pairs of numbered chromosomes (called autosomes) plus one pair of sex chromosomes. The notation for a typical female karyotype is 46,XX, and for a typical male it’s 46,XY.
How a Karyotype Image Is Made
To produce a karyotype image, a lab needs cells that are actively dividing, because chromosomes are only visible under a microscope during cell division. Technicians collect a sample, culture the cells, then stop division at the stage where chromosomes are most condensed and easiest to see. They stain the chromosomes with a dye called Giemsa, which creates a signature pattern of dark and light bands on each chromosome. Dark bands mark regions where DNA is more tightly packed and contains fewer active genes, while light bands correspond to more gene-rich, loosely packed regions.
Once stained, a technician photographs the chromosomes from a single cell, digitally cuts each one out, and lines them up next to their matching partner. The result is a karyogram: a neatly ordered portrait of all 46 chromosomes arranged from largest (chromosome 1) to smallest (chromosome 22), with the sex chromosomes placed last. A related term you might see is “idiogram,” which is a standardized diagram of a karyotype drawn to illustrate the banding pattern of each chromosome for a given species.
What Samples Are Used
The sample depends on the situation. For most children and adults, a standard blood draw provides enough white blood cells (lymphocytes) to culture and analyze. Skin cells called fibroblasts can also be used. In cancers like leukemia, bone marrow samples are routinely karyotyped to look for chromosome changes driving the disease.
During pregnancy, karyotyping can happen earlier than many people expect. Chorionic villus sampling (CVS) can be done as early as 10 weeks of gestation, using cells from the placenta. Amniocentesis, which collects cells floating in the amniotic fluid, is typically performed between weeks 15 and 18. Amniocentesis can also help confirm or rule out findings from CVS, since placental cells occasionally show chromosome patterns that don’t reflect the actual fetus.
What a Karyotype Can Detect
Karyotyping is designed to catch large-scale chromosome problems: missing or extra chromosomes, big chunks of DNA that have broken off and reattached in the wrong place, or segments that have flipped orientation. These categories cover a wide range of clinically significant conditions.
The most familiar example is Down syndrome, caused by an extra copy of chromosome 21. A person with Down syndrome typically has 47 chromosomes instead of 46, and their karyotype reads 47,XX,+21 or 47,XY,+21 depending on sex. Turner syndrome, which affects females, shows up as a missing X chromosome (45,X). Klinefelter syndrome in males involves an extra X chromosome (47,XXY). Rarer combinations exist too. A case report in the Journal of Pediatric Genetics documented a patient with both Down and Klinefelter syndromes, carrying a karyotype of 48,XXY,+21.
Beyond these well-known conditions, karyotyping picks up structural rearrangements like translocations (where pieces of two chromosomes swap places), inversions (where a segment within a chromosome flips around), and large deletions or duplications. In blood cancers, certain translocations visible on a karyotype are so strongly linked to specific cancer types that they guide treatment decisions.
What a Karyotype Cannot Detect
Standard karyotyping has a resolution limit of roughly 5 to 10 million base pairs of DNA per visible band. That sounds like a lot, and it is. Each band can contain hundreds of genes, which means changes smaller than about 3 to 5 million base pairs (megabases) are invisible on a karyotype. Single-gene mutations, the kind responsible for conditions like cystic fibrosis or sickle cell disease, are far too small to see this way.
Very subtle rearrangements, sometimes called cryptic or submicroscopic changes, can also slip through undetected and produce a false-negative result. If a chromosome has a tiny deletion or duplication below that 3 to 5 megabase threshold, the banding pattern looks normal even though DNA is missing or duplicated. Complex rearrangements involving multiple chromosomes and unusual marker chromosomes (small, unidentifiable chromosome fragments) can also be difficult to interpret with standard karyotyping alone.
Karyotyping vs. Chromosomal Microarray
Chromosomal microarray analysis (CMA) is a newer technology that scans the entire genome for gains and losses of DNA at a much finer resolution. While a karyotype misses deletions and duplications smaller than 3 to 5 megabases, microarray can detect imbalances smaller than 100 kilobases, roughly 30 to 50 times more sensitive. A 2024 study comparing both methods on 491 amniotic fluid samples confirmed that microarray catches microdeletions and microduplications that karyotyping simply cannot see.
That doesn’t make karyotyping obsolete, though. Microarray has its own blind spots. It cannot detect balanced rearrangements, where chromosome pieces swap places without any DNA being gained or lost. Balanced translocations and inversions look perfectly normal on microarray because the total amount of genetic material hasn’t changed. Karyotyping catches these because the technician can visually see that a chunk of chromosome 9 is sitting on chromosome 22, for instance, even if no DNA was lost in the exchange. For this reason, the two tests are often complementary rather than interchangeable.
How Long Results Take
Because cells need to be cultured before they can be analyzed, karyotype results are not instant. Turnaround time ranges from a few days to several weeks depending on the sample type and the lab. Prenatal results generally come back faster, since timing matters for pregnancy-related decisions. Postnatal blood samples may take longer because labs batch their work and the clinical urgency is often lower.
Common Reasons for Ordering a Karyotype
Doctors order karyotyping in a variety of situations. In newborns, it’s used when physical features suggest a chromosomal condition. In children, it may be requested to investigate developmental delays or ambiguous physical development. Couples experiencing repeated miscarriages are sometimes karyotyped to check for balanced translocations that, while harmless to the carrier, can cause unbalanced chromosomes in a pregnancy. In oncology, karyotyping bone marrow cells helps classify blood cancers and track how well treatment is working.
Prenatal karyotyping remains one of the most common uses. If a screening test during pregnancy flags an increased risk for a chromosomal condition, a diagnostic karyotype from CVS or amniocentesis provides a definitive answer. Unlike screening tests that estimate probability, a karyotype directly visualizes the chromosomes and confirms whether an extra or missing chromosome is present.