how many chromosomes in down syndrome

People with Down syndrome have 47 chromosomes in each cell instead of the usual 46.

Quick Scoop: Core Answer

• Typical humans have 46 chromosomes, arranged in 23 pairs.
• In Down syndrome (also called Trisomy 21), there is an extra copy of chromosome 21, so the total becomes 47 chromosomes.
• That extra chromosome 21—three copies instead of the usual two—is what causes the condition known as Down syndrome.

Mini Breakdown: Why 47?

• In most people:
  • 23 chromosomes come from the egg and 23 from the sperm → 46 total.
• In Down syndrome:
  • An error in cell division leads to an egg or sperm having an extra chromosome 21.
  • When fertilization happens, the baby ends up with 47 chromosomes (three number 21s).

Quick Forum-Style Note

When people online ask “how many chromosomes in Down syndrome,” they’re usually checking if it’s 46 or 47. The clear medical answer: 47 chromosomes, because of an extra chromosome 21.

TL;DR: A person with Down syndrome has 47 chromosomes, due to an extra copy of chromosome 21 (Trisomy 21).