Robertsonian translocation

Robertsonian translocation (also centric fusion or whole arm fusion) is a type of chromosomal translocation occurring mainly between acrocentric chromosomes, where the breakage points are between the short and long arms of two acrocentric chromosomes (which may or may not be homologous), such that the two long arms fuse and the two short arms fuse (the short arm fusion contains nonessential genes, such as those coding for rRNA which exist in multiple copies in the genome anyway, and as such the short arms are usually lost after a few rounds of cell division).

In balanced form, a Robertsonian translocation simply produces one fused chromosome with the full function of the two acrocentric chromosomes which gave rise to it, and as such there are no ill phenotypic effects. However, in unbalanced form, it can lead to imbalances in chromosome number and various syndromes of malformation and mental retardation. For instance, when unbalanced robertsonian translocation occurs between chromosomes 14 and 21 or 21 and 22 (as it commonly does), it can lead to trisomy 21 (Down's Syndrome, in this case referred to as translocation Downs). This is due to non-disjunction during gametogenesis and the mother has a higher risk of transmission (10%) than the father (1%).

People with Robertsonian translocations have only 45 chromosomes in each of their cells, yet all essential genetic material is present, and they appear normal. Their children, however, may either be normal and carry the fusion chromosome (depending which chromosome is represented in the gamete), or they may inherit a missing or extra long arm of an acrocentric chromosome. Unbalanced translocations can lead to uniparental disomy.