Mosaic Down syndrome is a condition where some, but not all, of a person's cells have an extra copy of chromosome 21.
Understanding Mosaic Down Syndrome
Most individuals have 46 chromosomes arranged in 23 pairs within each cell. These chromosomes contain the genetic information that guides growth and development. Down syndrome typically occurs when there's a full extra copy of chromosome 21 (trisomy 21) in all cells. However, in mosaic Down syndrome, the extra chromosome 21 is only present in a proportion of the cells.
How Does Mosaicism Occur?
Mosaicism generally happens after fertilization. Imagine a typically developing embryo with the standard 46 chromosomes. Early in cell division, an error can occur where one cell ends up with an extra copy of chromosome 21. From that point on, this cell (and all cells derived from it) will carry the extra chromosome, while other cells in the body will maintain the normal chromosome count. The percentage of cells with the extra chromosome 21 can vary widely among individuals with mosaic Down syndrome.
Implications and Variability
The implications of mosaic Down syndrome can differ significantly from person to person, largely depending on the proportion of cells with the extra chromosome. Individuals with a higher percentage of affected cells might exhibit more characteristics typically associated with Down syndrome. Conversely, those with a lower percentage of affected cells may have milder or fewer characteristics.
Diagnosis
Diagnosis usually involves a chromosome analysis (karyotype) performed on a sample of blood or other tissue. Because the extra chromosome 21 is not present in all cells, multiple samples may need to be tested to determine the level of mosaicism.
Characteristics
While the characteristics of mosaic Down syndrome can vary widely, potential features may include:
- Characteristic facial features
- Intellectual disability (ranging from mild to moderate)
- Heart defects
- Other health issues associated with Down syndrome
Because the symptoms can vary widely, it's best to consult with a geneticist for a comprehensive diagnosis and individualized treatment plan.
