Extra/missing chromosomes are the leading cause of mental retardation, pregnancy loss and IVF failure.  Most arise in the egg however the mechanism by which errors occur is under debate.  Normally, each chromosome of a pair migrates to opposite cell poles before cell division (disjunction); the textbook dogma suggests that errors occur when two chromosomes migrate to the same pole (nondisjunction). Angell and colleagues however proposed that, in fact, errors occurred when 1.5 chromosomes migrate to one pole and half a chromosome (chromatid) migrates to the other (precocious separation).  This model was widely respected, but not universally accepted, as critics purported that the observations could be down to technical artefacts.  Here we revisited this model using microarray technology and found that precocious separation is over 15 times more common than non-disjunction. (By Darren K Griffin, PhD, DSc, FIBiol, FRCPath, http://jmg.bmj.com/content/early/2011/05/26/jmg.2010.088070.abstract?papetoc )