47,XX,+21: What the Trisomy 21 Karyotype Means
Last updated: October 22, 2026
47,XX,+21
47,XX,+21 is the karyotype of a female with trisomy 21, the chromosome change that causes Down syndrome. This page breaks down the notation, shows what to look for when you karyotype the case, and covers the related results you will meet in practice.
What 47,XX,+21 means
| Part | Meaning |
|---|---|
| 47 | 47 chromosomes in each cell analyzed, one more than the usual 46 |
| XX | Two X chromosomes: the person is chromosomally female |
| +21 | The extra chromosome is a whole chromosome 21, so there are three copies of it (trisomy 21) |
The male equivalent is 47,XY,+21. Both are the karyotype of Down syndrome caused by a free trisomy, meaning the extra 21 is a separate chromosome rather than attached to another one.
What to look for in the karyotype
- A count of 47 in every cell, or in some cells if the case is mosaic.
- Three chromosomes 21 in the G group. Chromosome 21 is the smallest autosome, and its long arm looks dark because of a large dark band near the centromere (21q21).
- Two normal 22s. Chromosome 22 is slightly longer and mostly pale, with only a small dark band near the centromere. Mistaking a 22 for a 21 is the classic error in this case.
- In a male, the Y. The Y is also G-group size, but it has no satellites and its long arm varies in length and often looks fuzzy.
Check the count of acrocentrics. If you find three copies of 21q but a count of 46, look for a Robertsonian translocation: one of the D-group or G-group chromosomes will be a joined chromosome carrying an extra 21q.
How trisomy 21 happens
About 95 percent of Down syndrome is free trisomy 21. It is caused by nondisjunction, where the two chromosomes 21 fail to separate during meiosis, most often in the mother's first meiotic division. The chance of nondisjunction rises with maternal age. Free trisomy 21 is almost always a new event, not inherited.
About 3 to 4 percent is translocation Down syndrome, where the extra 21q is attached to another acrocentric chromosome, most often 14. A parent may carry the balanced form of that translocation, so the parents' chromosomes are usually checked. The remaining 1 to 2 percent is mosaic, with a mixture of trisomic and normal cells.
Related karyotypes
| Karyotype | Meaning |
|---|---|
| 47,XY,+21 | Male with free trisomy 21 |
| 46,XX,rob(14;21)(q10;q10),+21 | Translocation Down syndrome: 46 chromosomes, but three copies of 21q |
| 46,XY,rob(21;21)(q10;q10),+21 | Translocation Down syndrome with the two 21q arms joined together |
| 45,XX,rob(14;21)(q10;q10) | A balanced carrier of the 14;21 translocation, usually unaffected |
| mos 47,XX,+21[10]/46,XX[20] | Mosaic trisomy 21: 10 trisomic cells and 20 normal cells |
About Down syndrome
Down syndrome is the most common chromosomal condition in live births, occurring in roughly 1 in 700 births. It is associated with intellectual disability that varies widely between people, low muscle tone, characteristic facial features, and a higher risk of congenital heart defects, which affect about half of babies with the condition. People with Down syndrome also have a higher risk of thyroid disease, childhood leukemia and early-onset Alzheimer's disease. With modern care, most live well into adulthood.
Common questions
Does 47,XX,+21 always mean Down syndrome?
Yes. A karyotype showing full trisomy 21 in all cells is diagnostic of Down syndrome. A mosaic result means only some cells carry the extra chromosome, and the effects can be milder.
What does the plus sign mean?
In ISCN, a plus sign before a chromosome number means an extra copy of that whole chromosome. A minus sign means a missing one.
Can someone with 46 chromosomes have Down syndrome?
Yes, in translocation Down syndrome. The extra 21q is joined to another chromosome, so the count stays at 46 even though there are three copies of 21q.
Keep learning
This page is for education and training. It is not medical advice and must not be used to diagnose or treat anyone. Laboratory results should be interpreted by qualified professionals following current standards.