Subject: How can genetic mutations change the n chromosome number of a species?

how can a mutation provide an animal with an extra set of chromosomes? 
and, secondarily:
if specie with different numbers of chromosomes can't intermingle, how could an 
individual that ended up with an extra set of chromosomes reproduce?  i am 
assuming we are dealing with a sexually-reproducing animal here, because a 
plant would provide us with the possibility of self-pollination, and an asexual 
organism wouldn't need a counterpart to reproduce.  also, if this would depend 
on two individuals gaining an extra set of chromosomes at the same time and 
mating with each other, then how likely is this to occur?  obviously, at some 
time in the past, something of the sort has happened, for there to exist the 
variety of specie we have.  life didn't start at 46 cchromosomes.

Re: How can genetic mutations change the n chromosome number of a species?