There are three types of Down syndrome, identified based on chromosome studies done at birth to confirm the diagnosis:

1. Trisomy 21 (Nondisjunction): This is the most common type, accounting for 95% of cases. It occurs when a pair of 21st chromosomes in either the sperm or egg fails to separate before or during conception, leading to an extra chromosome 21 in every cell.
2. Translocation: This type accounts for 3% of cases. It occurs when part of chromosome 21 breaks off during cell division and attaches to another chromosome (usually chromosome 14). While the total number of chromosomes remains at 46, the extra piece of chromosome 21 leads to the characteristics of Down syndrome.
3. Mosaicism: This rare form accounts for 2% of cases. It occurs when nondisjunction happens in only some of the initial cell divisions, leading to a mixture of cells with 46 or 47 chromosomes. Individuals with mosaic Down syndrome may have fewer characteristics than those with other types, but every individual’s experience is unique.

Regardless of the type, the effects of Down syndrome vary from person to person, and each individual has their own strengths, preferences, and personality.