What type of chromosomal mutation causes down syndrome?

Down syndrome is primarily caused by a chromosomal mutation known as trisomy 21, where an individual has three copies of chromosome 21 instead of the usual two. This extra genetic material alters development and leads to the characteristic features and challenges associated with the condition. Most cases occur due to nondisjunction during meiOSis, where chromosomes fail to separate properly, resulting in gametes with an abnormal number of chromosomes.