mosaic Down syndrome

Mosaic Down syndrome is a rare form of Down syndrome where only some of an individual's cells have an extra copy of chromosome 21. This contrasts with the most common form, trisomy 21, where all cells have the extra chromosome. Because individuals with mosaic Down syndrome have some normal cells, the effects of the condition, such as intellectual disability, are typically less severe compared to those with trisomy 21.1

Down syndrome itself is a congenital disorder caused by the presence of extra genetic material from chromosome 21. This can manifest as an extra part of chromosome 21 or an entire extra copy, leading to a range of physical and cognitive impacts, from mild to severe. Common physical signs include a flattened face, up-slanted eyes, and poor muscle tone, while intellectual disability is present in all individuals with Down syndrome.12