The genetic basis of Down syndrome

Illustration showing the chromosomes of someone with Down syndrome

In humans, the egg cells and sperm cells have 23 chromosomes. The rest of your cells normally contain 23 pairs of chromosomes (a total of 46 chromosomes) — one set from your father and one set from your mother. Children with Down syndrome usually have three copies of chromosome 21 — called trisomy 21 — instead of two copies (a total of 47 chromosomes).

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