Which is the most common type of Down Syndrome?

• A. Trisomy 21
• B. Translocation Down Syndrome
• C. Mosaic Down Syndrome
• D. Partial Down Syndrome

Answer: (A)

The most common type of Down Syndrome is Trisomy 21. This condition occurs when an individual has three copies of chromosome 21 instead of the usual two. The presence of an extra chromosome results in the physical and developmental characteristics associated with Down Syndrome, including distinct facial features, potential cognitive delays, and various health issues. Trisomy 21 accounts for about 95% of all cases of Down Syndrome, making it the predominant type.

In contrast, translocation Down Syndrome involves a portion of chromosome 21 becoming attached (translocated) to another chromosome, which may occur in about 4% of Down Syndrome cases. Mosaic Down Syndrome, where some cells have the usual two copies of chromosome 21 while others have three, is even less common, occurring in about 1% of cases. Partial Down Syndrome is not widely recognized as a formal classification and is not a commonly referenced type. Understanding these different forms highlights why Trisomy 21 is the most prevalent and widely studied variant of Down Syndrome.