Robertsonian translocation

Robertsonian translocation (ROB) is a chromosomal abnormality where the entire long arms of two different chromosomes become fused to each other. It is the most common form of chromosomal translocation in humans, affecting 1 out of every 1,000 babies born.^[1] It does not usually cause medical problems, though some people may produce gametes with an incorrect number of chromosomes, resulting in a risk of miscarriage. In rare cases this translocation results in Down syndrome and Patau syndrome.^[2] Robertsonian translocations result in a reduction in the number of chromosomes. A Robertsonian evolutionary fusion, which may have occurred in the common ancestor of humans and other great apes, is the reason humans have 46 chromosomes while all other primates have 48. Detailed DNA studies of chimpanzee, orangutan, gorilla and bonobo apes has determined that where human chromosome 2 is present in our DNA in all four great apes this is split into two separate chromosomes typically numbered 2a and 2b.^[3][4] Similarly, the fact that horses have 64 chromosomes and donkeys 62, and that they can still have common, albeit usually infertile, offspring,^[5] may be due to a Robertsonian evolutionary fusion at some point in the descent of today's donkeys from their common ancestor.^[6]