Did you know, there are 3 types of Down Syndrome?
Trisomy 21
Roughly 95% have this type. This is when all of the cells have an extra copy of the 21st chromosome.
Mosaic Down Syndrome
(Also known as Mosaic Trisomy 21)
Roughly 2% have this type. This is when there is a set of typical cells, and another set of cells with an extra copy of the 21st chromosome. The % of these cells can vary from person to person. One person may have 5% of cells with it, while another could have 80% of cells with it. This will determine how many traits and characteristics of Trisomy 21 the person could receive.
Robertsonian Translocation:
Roughly 3% have this type. This is when the 21st chromosome is attached to another chromosome, rather than a complete extra chromosome of its own.
________________________________________
Common Questions:
How does having an extra 21st chromosome affect people?” and “Why are certain health conditions and facial features associated with Down Syndrome?”
When the cells of an individual has an extra 21st chromosome, certain genes may become over expressive. These over expressive genes are what can give someone the phenotypes (facial features) associated with this syndrome, as well as a higher susceptibility for congenital heart disease, thyroid disorders, intellectual disabilities, gastrointestinal disorders, etc...
People who have Mosaic Down Syndrome can have less over expressive genes, depending on the % of mosaicism, than some people with Trisomy 21 may have. This is why some people with Mosaic Down Syndrome get their diagnosis later in life.