Translocations: pieces in the wrong place
A translocation happens when a segment of one chromosome breaks off and reattaches to a different chromosome. If no genetic material is gained or lost in the swap, it is called balanced — and a person carrying a balanced translocation is often completely healthy, because all their genes are present, just rearranged. The catch comes at the next generation: during meiosis, a balanced carrier can produce unbalanced gametes that have too much of one segment and too little of another, raising the risk of a child with extra or missing material.
A Robertsonian translocation is a special version where two whole chromosomes fuse at their centromeres into a single chromosome. When this involves chromosome 21, a carrier can pass on an effective extra copy of 21 — producing translocation Down syndrome. This is the inherited route to Down syndrome hinted at earlier, and it explains why some families have a recurrence risk that the standard, age-related trisomy does not carry.
Translocations are not only inherited. An acquired translocation in a single cell line can drive cancer. The famous Philadelphia chromosome is a swap between chromosomes 9 and 22 that fuses two genes into one overactive growth signal, a defining feature of chronic myeloid leukemia. The same cytogenetic logic — broken ends rejoining in the wrong combination — runs from inherited conditions to acquired tumors.
Deletions, duplications, and microdeletions
A chromosomal deletion is a missing segment, so the genes within it are present in only one copy instead of two. A chromosomal duplication is the reverse: a doubled segment, giving three copies of those genes. Both are forms of copy number variation, and both shift gene dose for everything in the affected stretch — which is why even a small change can have outsized effects if the right genes are involved.
When a deletion is too small to see on a routine karyotype but still removes several neighbouring genes, the result is a microdeletion syndrome. Because the lost stretch spans multiple genes, these conditions present as a recognizable cluster of features rather than a single trait — and detecting them needs higher-resolution tools than the microscope alone, which is the subject of the next guide.
Structural changes at a glance:
Deletion ...[A B C D E]... --> ...[A B D E]... (lost C)
Duplication ...[A B C D E]... --> ...[A B C C D E]. (extra C)
Translocation chr-A: [..X..] chr-B: [..Y..]
--> chr-A: [..Y..] chr-B: [..X..]
Balanced translocation: all material present, just relocated
-> carrier usually healthy, but gametes may be unbalanced
Microdeletion: deletion too small for the karyotype to resolve
-> needs FISH or microarray to detect