A translocation is a type of chromosomal rearrangement where a piece of one chromosome breaks off and attaches to a different chromosome. It is the most common structural chromosomal abnormality in humans. Some translocations cause no health problems at all, while others play a direct role in conditions like Down syndrome and certain cancers.
How a Translocation Happens
Your DNA is organized into 23 pairs of chromosomes. During normal cell processes, chromosomes occasionally break. Usually the cell repairs these breaks accurately, but sometimes a broken segment from one chromosome reattaches to a completely different chromosome. When two chromosomes swap segments with each other, the result is a translocation.
What matters most is whether any genetic material gets lost or gained during the swap. This divides translocations into two broad categories: balanced and unbalanced. In a balanced translocation, all the genetic material is still present, just rearranged. The person typically has no symptoms because no genes are missing or duplicated. In an unbalanced translocation, the rearrangement results in extra or missing genetic material, which can cause developmental delays, intellectual disability, or birth defects.
Reciprocal vs. Robertsonian Translocations
There are two main types of translocations, and they differ in which chromosomes are involved and how the swap occurs.
Reciprocal translocations happen when two non-matching chromosomes break and exchange fragments with each other. The result is two rearranged chromosomes, each carrying a piece of the other. When no genetic material is lost, the translocation is balanced, and the person usually has no physical effects. These are found across any of the 23 chromosome pairs.
Robertsonian translocations involve a specific group of chromosomes: numbers 13, 14, 15, 21, and 22. These chromosomes have a distinctive shape where the short arms are very small and contain little essential genetic information. In a Robertsonian translocation, two of these chromosomes fuse together near their centers, and the small arms are lost. Because those lost segments don’t carry critical genes, the person is usually healthy. The most common Robertsonian translocations involve chromosomes 13 and 14, or chromosomes 14 and 21.
The Link to Down Syndrome
Most cases of Down syndrome (about 95%) occur because a person has three full copies of chromosome 21 instead of two, a result of errors during cell division. But roughly 3% to 4% of Down syndrome cases are caused by a Robertsonian translocation, most often between chromosomes 14 and 21. In these cases, the extra chromosome 21 material is physically attached to chromosome 14 rather than existing as a separate chromosome. The effect on the child is the same: three copies’ worth of chromosome 21 genes, producing the features of Down syndrome.
This distinction matters for families. Standard Down syndrome from a cell division error is almost always a random event. Translocation Down syndrome, on the other hand, can be inherited. One parent may carry a balanced translocation between chromosomes 14 and 21 with no symptoms at all, yet pass along an unbalanced version to their child.
Translocations and Cancer
Certain translocations are closely tied to specific cancers because the chromosomal swap creates a new, hybrid gene that drives uncontrolled cell growth. The most famous example is the Philadelphia chromosome, found in chronic myeloid leukemia (CML). It forms when chromosomes 9 and 22 swap segments, fusing two genes together. The resulting hybrid gene produces a protein that is permanently switched on, forcing blood-forming stem cells to multiply without the normal brakes.
Other well-known cancer-linked translocations include a swap between chromosomes 14 and 18 in follicular lymphoma (the most common translocation found in human cancer), a rearrangement involving the MYC gene in Burkitt lymphoma, and specific translocations in Ewing sarcoma and prostate cancer. In each case, the translocation either activates a growth-promoting gene or creates a fusion protein that pushes cells toward malignancy. Unlike inherited translocations, these typically occur in a single cell during a person’s lifetime and are not passed to children.
Effects on Fertility and Pregnancy
Many people discover they carry a balanced translocation only after experiencing difficulty conceiving or repeated miscarriages. The carrier is healthy because their genes are all present, just rearranged. But when their cells divide to form eggs or sperm, the rearranged chromosomes don’t always separate evenly. This can produce eggs or sperm with too much or too little genetic material, leading to embryos that either fail to implant or miscarry.
The numbers vary depending on which chromosomes are involved and whether the carrier is male or female. Broadly, female carriers face a 10% to 20% chance of producing offspring with chromosomal imbalance from any given pregnancy, while male carriers face a 5% to 10% chance. One large study found that female carriers had about a 10% risk of having a child with an unbalanced translocation, compared to roughly 7% for male carriers. Miscarriage rates among translocation carriers range from about 27% to 44%, compared to a baseline rate of around 12% in the general population. Carriers identified through recurrent miscarriage had the highest rates, at roughly 44%.
These statistics can sound alarming, but most translocation carriers do eventually have healthy children. Genetic counseling helps couples understand their specific risks, which depend heavily on exactly which chromosomes are involved and where the breakpoints fall.
How Translocations Are Detected
The standard test for identifying a translocation is a karyotype, which involves growing cells in a lab, staining their chromosomes, and examining them under a microscope. A trained technician can visually spot rearranged chromosomes and identify which ones are involved. Karyotyping remains the gold standard for detecting balanced translocations.
A more targeted technique called FISH (fluorescence in situ hybridization) uses fluorescent probes that bind to specific chromosome regions. It’s especially useful for confirming known translocations, like the Philadelphia chromosome in leukemia, and can be performed faster than a full karyotype.
Chromosomal microarray, a newer technology that scans the entire genome for missing or extra DNA, is powerful for detecting unbalanced rearrangements. However, it cannot detect balanced translocations because no genetic material is actually gained or lost. A major study published in the New England Journal of Medicine confirmed that microarray caught all the unbalanced rearrangements that karyotyping found but missed balanced translocations entirely. This is why karyotyping is still essential when a balanced translocation is suspected.
Long-read DNA sequencing is an emerging approach that can map translocation breakpoints down to the exact nucleotide, something conventional methods struggle with. This precision helps predict whether a translocation disrupts an important gene, which is particularly useful during prenatal diagnosis when a balanced translocation is found and doctors need to assess whether it might cause problems. While not yet routine in most clinical labs, long-read sequencing is increasingly used as a complement to traditional testing.
Living With a Balanced Translocation
Carriers of balanced translocations are physically healthy in the vast majority of cases. Unless a breakpoint happens to land in the middle of an important gene, the rearrangement causes no symptoms. Many people carry balanced translocations their entire lives without ever knowing. The translocation only becomes apparent when it causes reproductive problems or when a child is born with an unbalanced version.
If you or a partner are identified as a carrier, genetic testing of embryos before implantation (during IVF) is one option that allows selection of embryos with a normal or balanced chromosome arrangement. Prenatal testing through amniocentesis or chorionic villus sampling can also detect whether a developing pregnancy has inherited an unbalanced translocation. The right path depends on individual circumstances, the specific chromosomes involved, and the couple’s preferences.