) translocation

Translocations arise when DNA double-strand breaks occur in two different chromosomes and are repaired incorrectly, joining segments from nonhomologous chromosomes. Two major types: (1) Reciprocal translocation - exchange of segments between two chromosomes at breakpoints, usually balanced with no clinical phenotype in the carrier but risk of unbalanced gametes leading to miscarriage or abnormal offspring. (2) Robertsonian translocation - fusion of two acrocentric chromosomes (13,14,15,21,22) at the centromere with loss of short arms; carriers are phenotypically normal but at risk for unbalanced gametes causing translocation Down syndrome (extra chromosome 21 material) or other trisomies. Balanced translocation carriers are typically asymptomatic but have increased risk of infertility, recurrent miscarriage, and translocation-associated malignancies if the breakpoint disrupts an oncogene or creates a fusion gene. Translocations are a major mechanism of oncogenesis by juxtaposing an oncogene next to a highly active promoter (e.g., MYC-IGH in Burkitt lymphoma, t(8;14)) or by creating a novel fusion protein with oncogenic activity (e.g., BCR-ABL in CML, t(9;22) 'Philadelphia chromosome', producing a constitutively active tyrosine kinase, targeted by imatinib; PML-RARA in acute promyelocytic leukemia, t(15;17), disrupting retinoic acid receptor signaling, treated with ATRA; EWS-FLI1 in Ewing sarcoma, t(11;22); IGH-BCL2 in follicular lymphoma, t(14;18) leading to BCL2 overexpression and apoptosis resistance). Diagnosis is via karyotype, FISH, or PCR. Clinically important for cancer diagnosis, prognosis, and targeted therapy selection, as well as prenatal genetic counseling in balanced translocation carriers.