Extra 21 chromosome
The majority of down syndrome
develops when the fertilized egg gets an extra number 21 chromosome from the mother. When that cell divides and forms an embryo, every one of the cells in the baby's body have that extra 21 chromosome.
There are rare cases where a baby inherits an extra part of the 21 chromosome from one of it's parents. The extra genes on the 21 chromosome cause the down syndrome.