Subject: Do the zygotes of the same population vary only in their genes?

I homeschool my two oldest children.  In biology class, when discussing 
heredity, I said that the only variation among the zygotes, or same sex 
gametes, of the same species was genetic, and that that is why all heritable 
variation within populations always reduces to variation in genes.  But now I 
can't find it anywhere that this is actually true.  None of our books say 
anything about the extra-genetic variation of zygotes, or sperma and ova, in 
the same population, so maybe what I said, and have always assumed, is wrong.  
Perhaps I am oversimplifying things. Of course, neither of my children really 
care much about biology; still, they have to take it, and I want to be correct.

Re: Do the zygotes of the same population vary only in their genes?