Robertsonian translocation
A Robertsonian translocation is a special fusion in which two whole chromosomes join end to end at their centromeres, forming one combined chromosome. Picture two short books bound together into a single spine. It only happens to the five human acrocentric chromosomes — 13, 14, 15, 21 and 22 — whose centromeres sit very near one tip.
Because these chromosomes lose only tiny, gene-poor stubs in the fusion, a carrier usually has 45 chromosomes yet is healthy: essentially all their important genes are still present in the right dose. The arrangement is therefore a balanced rearrangement, often discovered only when reproductive problems arise.
The reproductive consequences are the point. When a Robertsonian carrier forms eggs or sperm, the fused chromosome can segregate in ways that leave a gamete with an extra or missing chromosome. A carrier of a 14;21 fusion, for instance, has an elevated chance of a child with translocation Down syndrome, and such Down syndrome can recur across generations independent of maternal age.
A healthy parent with a balanced 13;14 Robertsonian translocation has 45 chromosomes but a raised risk of miscarriage and of a child with trisomy 13.
Centric fusion keeps a carrier healthy but unbalances some of their gametes.
Unlike free trisomy 21, translocation Down syndrome can be inherited from a balanced parent and may recur in a family regardless of the mother's age.