People with Down syndrome have 47 chromosomes in each cell instead of the usual 46. The extra chromosome is a third copy of chromosome 21, which is why the condition is also called trisomy 21. That single additional chromosome affects development throughout the body, influencing physical traits, learning, and health from birth onward.
Why 47 Instead of 46
Every human cell normally contains 23 pairs of chromosomes, totaling 46. You get one set of 23 from your mother’s egg and one set of 23 from your father’s sperm. In Down syndrome, something goes wrong during the formation of either the egg or sperm cell. A process called nondisjunction occurs when a pair of chromosomes fails to separate properly during cell division. The result is a reproductive cell that carries two copies of chromosome 21 instead of one. When that cell combines with a normal cell at fertilization, the embryo ends up with three copies of chromosome 21 and 47 total chromosomes.
This error happens before fertilization, during the specialized cell division that produces eggs and sperm. It is not caused by anything either parent did or didn’t do during pregnancy. The extra chromosome is present from the very first cell of the embryo and gets copied into every cell as the baby develops.
Three Types With Different Chromosome Patterns
Not every case of Down syndrome looks the same at the chromosomal level. There are three distinct types, and the chromosome count can differ between them.
Trisomy 21 is the most common form, accounting for about 95% of cases. Every cell in the body contains 47 chromosomes. This is the straightforward pattern most people think of.
Translocation Down syndrome makes up about 3% to 4% of cases. Here, the total chromosome count may appear to be the normal 46, but the extra chromosome 21 material is still present. It has broken off and attached itself to another chromosome, usually chromosome 14. So while a standard chromosome count looks normal, the person still carries three copies’ worth of chromosome 21 genetic material. This is the only type that can sometimes be inherited from a parent who carries the translocation without showing symptoms.
Mosaic Down syndrome is the rarest form, occurring in about 1% to 2% of cases. In this type, some cells have the typical 46 chromosomes while others have 47. The mix happens because the nondisjunction error occurs after fertilization, during early cell division of the embryo. People with mosaic Down syndrome may have fewer or milder features of the condition, depending on the proportion of cells affected.
How the Extra Chromosome Is Detected
A test called a karyotype is the definitive way to confirm Down syndrome. It examines cells from a blood sample, arranging all the chromosomes by size and shape so they can be counted and inspected visually. A karyotype clearly shows whether there are three copies of chromosome 21 and identifies which type of Down syndrome is present.
Before birth, screening can flag the likelihood of Down syndrome well before a karyotype is performed. A blood test that analyzes fragments of fetal DNA circulating in the mother’s bloodstream detects trisomy 21 with about 99.3% sensitivity and 99.9% specificity, according to a meta-analysis published in BMJ Open. This screening is typically offered in the first trimester. A positive screening result is then confirmed through amniocentesis or chorionic villus sampling, which collect fetal cells for a full karyotype.
How Common Down Syndrome Is
About 5,700 babies are born with Down syndrome each year in the United States, roughly 1 in every 640 births. It is the most common chromosomal condition diagnosed in newborns.
The chance of having a baby with Down syndrome rises with maternal age, and the increase is steep. At age 25, the probability is about 1 in 1,300. By age 35, it jumps to 1 in 365. At age 45, the risk reaches approximately 1 in 30. The reason is that a woman’s egg cells age along with her, and older eggs are more prone to the nondisjunction error that produces the extra chromosome. That said, most babies with Down syndrome are born to women under 35 simply because younger women have more pregnancies overall.
What the Extra Chromosome Means Day to Day
Chromosome 21 is one of the smallest human chromosomes, but carrying an extra copy affects many systems. The additional genetic material alters how proteins are produced during development, leading to a recognizable set of physical features: a flattened facial profile, upward-slanting eyes, a single deep crease across the palm, and lower muscle tone at birth. Not every person with Down syndrome has all of these traits, and the features vary widely in how noticeable they are.
Intellectual disability ranges from mild to moderate. Most children with Down syndrome learn to walk, talk, read, and participate in school, though they typically reach milestones later than their peers. Early intervention programs that begin in infancy, including speech therapy, physical therapy, and occupational therapy, significantly improve outcomes.
Certain health conditions are more common because of the extra chromosome’s influence on development. About half of babies with Down syndrome are born with a heart defect, many of which can be repaired surgically. Thyroid problems, hearing difficulties, and vision issues also occur at higher rates and are monitored throughout life. Life expectancy has increased dramatically over the past several decades, largely due to advances in heart surgery and medical care, and now averages around 60 years.