The gonadal translocation would end up in an egg or sperm which fused with a gamete of the opposite type to produce the child with Down syndrome. These joined chromosomes act as a single one and join to the other gamete's normal chromosome 21, resulting in three such chromosomes in the child (hence the name trisomy 21).
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This is a second way to produce the gamete with an extra copy of chromosome 21. The more common cause of Down syndrome is nondisjunction during meiosis which creates one gamete missing chromosome 21 and another with a third attached copy.