Down Syndrome is usually caused by an extra copy of which chromosome?

The story behind the answer

Down syndrome is most commonly caused by trisomy 21—having three copies of chromosome 21 instead of the usual two. This extra genetic material changes typical development and can lead to characteristic facial features, developmental delays, and increased risk of conditions like congenital heart defects and thyroid issues. Most cases arise from nondisjunction during meiosis, when chromosomes fail to separate properly; less commonly, it results from translocation or mosaicism.

Memory tip: World Down Syndrome Day is March 21 (3/21), symbolizing three copies of chromosome 21. Remember “21 for 21” to link the condition to its chromosomal cause.