On average, what percentage of infants born to 45-year-old mothers have down syndrome

Probably 3 %. a mother could give birth to a child with Trisomy 21 at any age because Trisomy 21 is a mutation and mutations happen all the time. Not all mutations are as Down Syndrome. A child born to two homozygous brown eyed parents could be born with blue eyes even if no one in the family had blue eyes. The child would develop blue eyes due to mutation.

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