The extra 21st chromosome that is found with Down syndrome usually comes from the father's sperm.

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The extra 21st chromosome that causes Down syndrome typically comes from the mother's egg. This occurs due to a phenomenon called non-disjunction, where the egg or sperm does not divide properly during meiosis and ends up with an extra copy of chromosome 21. While it is possible for the extra chromosome to come from the father's sperm, it is much less common than from the mother's egg.

The Extra 21st Chromosome That Is Found with Down Syndrome