I am still only halfway through 'A World of Iris', but this is what I found out Neal. Dwarves and TBs are fertile. Intermediates have sterile pollen that doesn't undergo mitosis correctly (there are lags). It took awhile to find this out because the pollen looks fine to the visible eye. The IBs are partially fertile with SDBs as TBs, but not with IBs. Offspring of IBs can separate normal traits like it can produce SDB offspring that bloom during TB season etc. Mid-America's new SDBs are the result of TB pollen introducing the modern TB traits to the dwarves.
