The karyotype of Down syndrome is 47,XX,+21 in females and 47,XY,+21 in males. That notation tells you everything essential: the person has 47 chromosomes instead of the typical 46, and the extra one is a third copy of chromosome 21. This is why Down syndrome is also called trisomy 21. However, not every case looks exactly the same on a karyotype report. There are three genetic types, each with its own notation.
Standard Trisomy 21
About 95% of people with Down syndrome have the straightforward form: a complete extra copy of chromosome 21 in every cell. When a lab lines up and photographs the chromosomes, you can literally see three copies sitting in the chromosome 21 slot instead of two. The written result reads 47,XX,+21 for a female or 47,XY,+21 for a male. The “47” is the total chromosome count, the sex chromosomes come next, and “+21” specifies which chromosome has an extra copy.
This type happens because of a cell division error called nondisjunction. During the formation of an egg or sperm, the pair of chromosome 21s fails to separate properly, so one reproductive cell ends up with two copies and the other with none. When the cell carrying two copies joins with a normal cell at fertilization, the resulting embryo has three. Research analyzing DNA markers in 188 maternal cases found that about 77% of the time, this error occurs during the first stage of egg cell division, while about 23% happen during the second stage. In the small number of cases traced to the father’s sperm, the pattern flips: roughly 78% of errors occur during the second division stage.
Translocation Down Syndrome
In about 3 to 4% of cases, the total chromosome count is 46, not 47, yet the person still has Down syndrome. This happens when the extra chromosome 21 material isn’t floating free as a separate chromosome. Instead, it’s physically attached to another chromosome, most commonly chromosome 14. The karyotype notation for this is 46,XX,t(14;21) or 46,XY,t(14;21).
This form matters for family planning because it can be inherited. In some cases the translocation arises brand new in the child. But in others, a parent carries a “balanced” version of the translocation: they have the right amount of genetic material overall, just rearranged, so they’re perfectly healthy. That parent, however, has an increased chance of having another child with Down syndrome. When translocation is found on a child’s karyotype, the parents are usually offered testing to determine whether one of them is a carrier.
Mosaic Down Syndrome
Mosaic Down syndrome accounts for about 1 to 2% of cases. Here, only some cells have the extra chromosome 21 while others have the typical 46. The karyotype report lists both cell lines, something like 47,XX,+21[10]/46,XX[40], where the numbers in brackets show how many cells of each type were counted in the sample.
The percentage of affected cells can vary dramatically, and it often differs depending on which tissue is tested. In one documented case, a baby girl had 8 to 13% trisomy cells in her blood but 31% in cells swabbed from the inside of her cheek. Another case involved a 14-year-old girl with just 2% trisomy cells in blood and 11% in cheek cells. Because blood testing alone can miss low-level mosaicism, labs sometimes analyze cells from multiple tissues. In one case report, standard blood chromosome analysis found only 1 trisomic cell out of 50, but a specialized test on cheek cells detected three copies of chromosome 21 in about 15.6% of cells, confirming the diagnosis.
People with mosaic Down syndrome generally have milder features, though there’s wide individual variation that doesn’t always correlate neatly with the percentage of trisomic cells in any single tissue sample.
How a Karyotype Is Created
A karyotype isn’t just a genetic test you get results from. It’s a physical image of every chromosome in a cell, arranged by size and banding pattern. Producing one takes several days of laboratory work.
The process starts with a sample of living cells, typically from blood, amniotic fluid, or placental tissue. These cells are cultured so they begin dividing. At just the right moment, the lab adds a chemical that freezes the cells mid-division, when chromosomes are most tightly coiled and easiest to see. The cells are then swollen with a salt solution, preserved with a fixative, and dropped onto glass slides.
To tell the chromosomes apart, the lab uses a staining technique called G-banding. An enzyme partially digests the chromosome surfaces, then a dye called Giemsa is applied. This produces a unique pattern of light and dark horizontal stripes on each chromosome, like a barcode. A technician examines the chromosomes under a microscope at high magnification, photographs them, and arranges them into numbered pairs. Chromosome 21 is one of the smallest, and spotting a third copy is straightforward for a trained eye.
Karyotype vs. Prenatal Screening
If you’ve had prenatal screening through a blood draw (cell-free DNA screening, sometimes called NIPT), you may wonder how that compares to a karyotype. The short answer: NIPT is a screening test, while karyotyping is a diagnostic test. NIPT analyzes fragments of fetal DNA circulating in the mother’s blood and is highly accurate for detecting trisomy 21, with sensitivity around 96%. But it can produce false positives and occasionally false negatives, particularly for mosaic cases.
A karyotype performed on fetal cells collected through amniocentesis or chorionic villus sampling has 100% sensitivity for aneuploidy. It’s the definitive confirmation. So when NIPT flags a high risk for Down syndrome, the next step is typically an invasive procedure to obtain fetal cells for a full karyotype, which also reveals whether the trisomy is standard, translocation, or mosaic.
Why Maternal Age Affects Risk
The risk of having a baby with trisomy 21 rises with the age of the mother at conception, particularly after age 35. This is because a woman’s eggs have been suspended mid-division since before she was born. The longer they sit in that paused state, the more likely the chromosome pairs are to separate incorrectly when division finally resumes decades later. That said, the majority of babies with Down syndrome are born to mothers under 35, simply because younger women have far more pregnancies overall.
Paternal age plays a smaller role. The errors in sperm cell division that lead to trisomy 21 are much less common, accounting for only a small fraction of cases.