A karyotype is the complete set of chromosomes in a person’s cells, or more commonly, a lab-produced image of those chromosomes arranged in order by size and shape. Humans normally have 46 chromosomes organized into 23 pairs. A karyotype lets doctors spot missing, extra, or rearranged chromosomes that can cause genetic conditions or guide cancer treatment.
How a Karyotype Is Made
Creating a karyotype starts with collecting living cells, most often from a blood sample but also from bone marrow, skin, amniotic fluid, or tumor tissue. Those cells are placed in a nutrient-rich culture medium and grown in an incubator at body temperature. The goal is to catch cells in the brief moment during cell division when chromosomes are at their most compact and visible under a microscope.
To freeze cells at exactly that point, the lab adds a chemical that stops the internal machinery cells use to pull chromosomes apart. The cells are then placed in a salt solution that causes them to swell, spreading the chromosomes out so they don’t overlap. A fixative locks them in that swollen state, and the cell mixture is dropped onto a glass slide from a few inches above, at an angle, so the chromosomes scatter evenly.
The final step is staining. A technique called G-banding uses an enzyme to partially digest the chromosome surface, followed by a dye that creates a signature pattern of light and dark horizontal bands on each chromosome. These banding patterns are like a barcode: every chromosome has a unique one, making it possible to tell chromosome 4 from chromosome 5, or to see when a piece of one chromosome has broken off and attached to another. A technician then photographs the stained chromosomes, digitally clips each one out, and lines them up from largest (chromosome 1) to smallest (chromosome 22), with the sex chromosomes (X and Y) placed last.
What a Normal Karyotype Looks Like
A typical female karyotype is written as 46,XX and a typical male as 46,XY. The first number is the total chromosome count, and the letters indicate which sex chromosomes are present. Each of the 22 numbered pairs (called autosomes) should have two copies of matching size, banding pattern, and centromere position, which is the pinched-in point that divides a chromosome into its two arms.
Conditions a Karyotype Can Detect
Karyotyping is particularly good at finding whole extra or missing chromosomes. Down syndrome, for example, shows up as three copies of chromosome 21 instead of two, written as 47,XX,+21 or 47,XY,+21. Turner syndrome appears as a missing sex chromosome in females (45,X), while Klinefelter syndrome, the most common chromosomal aneuploidy, shows an extra X in males (47,XXY). More than 90% of Klinefelter cases have that specific pattern, though rarer variants with additional X chromosomes also occur.
Beyond extra or missing chromosomes, karyotyping reveals large structural problems: pieces of chromosomes that have broken off and reattached to the wrong partner (translocations), segments that are flipped in orientation (inversions), or chunks that are missing entirely (deletions). These rearrangements can cause infertility, recurrent miscarriage, or developmental differences depending on which genes are involved.
Karyotyping During Pregnancy
Prenatal karyotyping remains the definitive way to diagnose chromosomal conditions in a fetus. Two procedures can provide the cells needed. Chorionic villus sampling (CVS) takes a tiny biopsy of placental tissue, typically between 10 and 12 weeks of pregnancy. Amniocentesis draws a small amount of the fluid surrounding the fetus, which contains shed fetal cells, and is usually performed between 15 and 18 weeks, though some centers offer it as early as 11 weeks.
Both procedures carry a small risk of complications, so they’re generally offered when screening tests suggest an elevated chance of a chromosomal condition, when a parent carries a known rearrangement, or when the mother is over a certain age. The cells collected are cultured and karyotyped using the same process as a blood sample.
How Karyotypes Guide Cancer Treatment
In blood cancers like leukemia, the karyotype of the cancer cells is one of the single most important factors in predicting how well treatment will work. Doctors classify the chromosomal findings into favorable, intermediate, or unfavorable risk groups. Patients with favorable chromosome patterns tend to respond well to standard chemotherapy, while those with unfavorable patterns face higher relapse rates, and that information directly shapes decisions about treatment intensity and whether a stem cell transplant should be pursued.
Karyotyping also helps confirm whether a blood abnormality is truly cancerous or just a reactive process, track how well a patient is responding to treatment, and catch early signs of disease returning after remission. Bone marrow is the most common sample type for cancer karyotyping, though peripheral blood and tumor biopsies are also used.
How Long Results Take
Because cells need time to grow in culture before they can be analyzed, karyotype results are not instant. Blood and bone marrow samples typically take 3 to 14 days. Solid tissue samples from lymph nodes or tumors can take 5 to 21 days, since those cells may grow more slowly. Prenatal samples fall in a similar range depending on how quickly the fetal cells divide in culture.
What Karyotyping Cannot See
Standard karyotyping has a resolution limit of roughly 3 to 5 million base pairs of DNA. That means it can spot large-scale changes but will miss smaller deletions or duplications beneath that threshold. For context, some genetic conditions are caused by missing or duplicated stretches of DNA smaller than 100,000 base pairs, which is far below what a microscope-based technique can resolve.
When doctors suspect a smaller change, they turn to higher-resolution tools. Chromosomal microarray analysis can detect imbalances as small as 100,000 base pairs by scanning the entire genome at a molecular level. Another option, fluorescence in situ hybridization (FISH), uses fluorescent probes that bind to specific chromosome regions and can return results in 2 to 7 days, making it useful when a faster answer is needed for a known target. Neither of these replacements makes karyotyping obsolete, though. Karyotyping is still the only routine test that gives a full visual picture of all 46 chromosomes at once, revealing balanced rearrangements like translocations and inversions that microarray technology can miss entirely.