Since humans with monosomy 2 cannot survive, there must have been some point at which our ancestors could survive with this condition, otherwise how would we have got here? so do we know what it is about monosomy 2 that stops us from surviving it now?

Well as no-one has replied to this yet, I'll ask what's the mechanism (I'm presuming for fusion) that requires monosomy in chromosome 2?

 

All the momosomies, at least for autosomes, are practically lethal. Turner syndrome, where a woman has a single X chromosome, is fairly common (one is a few thousand by memory) but it results in sterility too. So it'd probably make more sense for some other means of fusion to occur.