translocation
A translocation is when a piece of one chromosome breaks off and reattaches to a different, non-matching chromosome — like tearing a chapter out of one book and gluing it into another. The DNA itself may all still be present, just relocated to the wrong volume on the shelf.
Translocations come in two main types. A reciprocal translocation swaps segments between two chromosomes; a Robertsonian translocation fuses two whole chromosomes at their centromeres. If no genetic material is gained or lost, the rearrangement is called balanced, and the carrier is often perfectly healthy because all their genes are still present and dosed correctly.
The catch is reproduction. A balanced carrier can produce eggs or sperm that are unbalanced, carrying extra or missing segments, which can lead to miscarriage or a child with a chromosomal disorder. Translocations are also central to cancer: certain swaps fuse two genes into a new hybrid that drives uncontrolled growth, as in the Philadelphia chromosome.
In the Philadelphia chromosome a piece of chromosome 9 and a piece of chromosome 22 swap places, fusing the BCR and ABL1 genes into a cancer-driving hybrid.
A reciprocal swap can be harmless when balanced — or fuse genes into a cancer driver.
Balanced translocations often cause no symptoms in the carrier but raise the chance of unbalanced gametes; that is why a healthy parent can still have an affected child.