Which Down Syndrome subtype occurs when one cell divides correctly and another divides incorrectly, resulting in some cells with 46 chromosomes and others with 47?

• A. Trisomy 21
• B. Translocation
• C. Mosaicism
• D. Monosomy 21

Answer: (C)

Mosaic Down syndrome comes from mosaicism, where two different cell lines exist in the same person. After fertilization, a mitotic error in one early cell division causes one daughter cell to acquire an extra chromosome 21 while its twin remains normal. As development continues, some tissues are made up of cells with 46 chromosomes and others with 47, creating a mix of cell populations. This explains why the condition can show variable features and sometimes appear milder, depending on how many cells and which tissues carry the extra chromosome. In contrast, trisomy 21 affects all cells with an extra chromosome, translocation Down syndrome involves extra material attached to another chromosome but often with a normal overall chromosome count, and monosomy 21 is typically not viable.