People with Down syndrome have 47 chromosomes instead of the typical 46. The extra chromosome is a third copy of chromosome 21, which is why the condition is also called trisomy 21. This small addition of genetic material affects development throughout the body, but the exact chromosome count can vary slightly depending on which type of Down syndrome a person has.
Why 47 Instead of 46
Human cells normally contain 23 pairs of chromosomes, totaling 46. You inherit one set of 23 from your mother’s egg and one set of 23 from your father’s sperm. In Down syndrome, an error during the formation of the egg or sperm causes one of them to carry two copies of chromosome 21 instead of one. When that egg or sperm joins with its partner at fertilization, the resulting embryo ends up with three copies of chromosome 21, bringing the total to 47.
This error is called nondisjunction. During cell division, the paired chromosomes are supposed to separate so each new cell gets one copy. In nondisjunction, both copies travel into the same cell, leaving the other cell with none. The cell with the extra chromosome 21 is the one that can lead to a pregnancy with Down syndrome.
Three Types, Different Chromosome Arrangements
Not every person with Down syndrome has the same chromosomal picture. There are three distinct types, and they differ in how the extra chromosome 21 material is distributed.
Complete Trisomy 21
This is the most common form, accounting for about 95% of cases. Every cell in the body contains 47 chromosomes, with three full copies of chromosome 21. The nondisjunction error happened before or at conception, so it carries through to every cell as the embryo grows.
Translocation Down Syndrome
In translocation cases, the total chromosome count may appear to be 46, but the person still has three copies’ worth of chromosome 21 material. What happens is that an extra copy of chromosome 21 breaks off and attaches itself to another chromosome, most often chromosome 14, though it can also attach to chromosomes 13, 15, or 22. A standard chromosome count might look normal at first glance, but closer analysis reveals the extra genetic material riding along on the other chromosome. In some cases, two chromosome 21 copies fuse together. This type makes up about 3 to 4% of Down syndrome cases and is the only form that can sometimes be inherited from a parent who carries the rearrangement without showing symptoms.
Mosaic Down Syndrome
Mosaic Down syndrome is the rarest form, occurring in about 1 to 2% of cases. Here, the nondisjunction error happens after fertilization, during one of the early cell divisions. The result is a mix: some cells have the typical 46 chromosomes, and others have 47. The ratio varies from person to person. To determine the level of mosaicism, a lab typically analyzes 20 cells. If 15 of those 20 cells show 47 chromosomes and the other 5 show 46, the mosaicism level would be 75%. People with lower percentages of affected cells sometimes have milder features, though this isn’t guaranteed.
What the Extra Chromosome Actually Does
Chromosome 21 is the smallest human chromosome, but it still carries hundreds of genes. When three copies are present instead of two, those genes produce roughly 50% more of their protein products than usual. Researchers have identified a specific region on the long arm of chromosome 21, located in a band called 21q22.3, that appears especially important. This stretch, estimated at somewhere between 400 and 3,000 kilobases of DNA, contains genes whose overexpression drives many of the characteristic features of Down syndrome, including intellectual disability, distinct facial features, and heart differences.
The extra genetic material doesn’t cause a single, uniform effect. It influences dozens of developmental pathways, which is why Down syndrome affects so many systems: heart structure, muscle tone, immune function, and cognitive development among them.
How the Extra Chromosome Is Detected
A karyotype test is the definitive way to confirm Down syndrome. A lab takes a sample of cells (from blood, amniotic fluid, or placental tissue), stains the chromosomes, photographs them, and arranges them by size and shape into their 23 pairs. An extra chromosome 21 is visible in the image, and translocation cases show the fused chromosomes clearly.
Before birth, screening can flag the possibility earlier. Non-invasive prenatal testing (NIPT), which analyzes fragments of fetal DNA circulating in the mother’s blood, detects trisomy 21 in 98 to 99% of affected pregnancies. The false-positive rate is low, about 1 in 500. A positive screening result is typically followed by a diagnostic test like amniocentesis or chorionic villus sampling to confirm the chromosome count directly.
How Common It Is
About 5,700 babies are born with Down syndrome in the United States each year, a rate of roughly 1 in every 640 births. The likelihood increases with maternal age. At ages 20 to 24, the chance is about 1 in 1,562. By ages 35 to 39, it rises to about 1 in 214. For mothers over 45, it reaches approximately 1 in 19. The sharp increase happens because eggs that have been stored in the ovaries for decades are more prone to the nondisjunction errors that produce an extra chromosome. That said, most babies with Down syndrome are born to mothers under 35, simply because younger women have far more pregnancies overall.
The father’s sperm can also be the source of the extra chromosome, though this accounts for a smaller proportion of cases. Translocation Down syndrome is the only type with a clear hereditary component; a parent can carry a balanced translocation (with no extra genetic material and no symptoms) and pass the unbalanced version to a child.