Trisomy 21
This is the most common type, accounting for 95% of cases of Down syndrome. Typically, children are born carrying 46 chromosomes in each cell (23 from the mother and 23 from the father). In Trisomy 21, an error in cell division called "nondisjunction" occurs, resulting in the embryo developing with an extra copy of chromosome 21 in every cell — three copies instead of the usual two.
Translocation
Translocation accounts for 4% of cases of Down syndrome. In this type, a portion of chromosome 21 breaks off and attaches to another chromosome, usually chromosome 14. While the total number of chromosomes remains 46, the presence of this extra segment of chromosome 21 is what causes Down syndrome.
Mosaic Down Syndrome
Mosaic Down syndrome occurs when there is an error in one of the first cell divisions after fertilization, but not in all subsequent divisions. As a result, some cells have the typical 46 chromosomes, while others have 47, carrying an extra copy of chromosome 21.
This type accounts for approximately 1% of cases of Down syndrome. Research has indicated that individuals with mosaic Down syndrome may exhibit fewer characteristics associated with the condition compared to other types. However, this cannot be generalized, given the wide variation in abilities and potential among individuals with Down syndrome.
Incidence of Down Syndrome
Down syndrome occurs in approximately one in every 700 births and is the most common chromosomal condition. It occurs across all civilizations, ethnic groups, and socioeconomic levels.
The likelihood of having a child with Down syndrome increases with the mother's age. As many couples delay having children until later in life, the chances of having a child with Down syndrome increase accordingly. This has made genetic counseling for parents increasingly important.
| Mother's Age | Incidence of Down Syndrome |
|---|---|
| 20 | 1 in 2,000 |
| 21 | 1 in 1,700 |
| 22 | 1 in 1,500 |
| 23 | 1 in 1,400 |
| 24 | 1 in 1,300 |
| 25 | 1 in 1,200 |
| 26 | 1 in 1,100 |
| 27 | 1 in 1,050 |
| 28 | 1 in 1,000 |
| 29 | 1 in 950 |
| 30 | 1 in 900 |
| 31 | 1 in 800 |
| 32 | 1 in 720 |
| 33 | 1 in 600 |
| 34 | 1 in 450 |
| 35 | 1 in 350 |
| 36 | 1 in 300 |
| 37 | 1 in 250 |
| 38 | 1 in 200 |
| 39 | 1 in 150 |
| 40 | 1 in 100 |
| 41 | 1 in 80 |
| 42 | 1 in 70 |
| 43 | 1 in 50 |
| 44 | 1 in 40 |
| 45 | 1 in 30 |
| 46 | 1 in 25 |
| 47 | 1 in 20 |
| 48 | 1 in 15 |
| 49 | 1 in 10 |
Source: The information in this article has been reproduced under an exclusive agreement with the National Down Syndrome Society. Available at: ndss.org
