White Spotting- genetic question

[Muridea]

I want to know has anyone had this happen, is this normal or not.  I have been selecting in one line for lots of white and roaning of colour on the back.  I began with a gerbil that just had a headspot and partial colour- not very impressive- yet in a few generation of careful selection i have gotten to gerbils with patches of colour just around the ears, the white colour extending to midbody and the remainer of the rump coloured but extreemly roaned so that it looks almost like a chinchilla colour.   Here is what I find odd though- Sp is dominant and homozygous lethal. I am breedign spotted to spotted without any problems.  Theoretically I SHOULD get some solid coloured gerbils (about 1/3rd the litter actually born) but I dont.  Every gerbil is either "extreem" spotted (as described above) or reverted to the original spotting pattern of the original spotted female - just a head spot with partial collar and no roaning of colour on the rump.  My litters also tend to be about 6 babies each but then again, so do most of my non-spotted gerbil's litters- I was expecting smaller litters but it hasnt happened....   Any idea why this happens (no solid coloured gerbils and no significant change in litter size)?  My first thought was a non-lethal variation of the SP allele.  My next idea was that i had overselected for spotting modifiers that when brought all together manage to make spotted gerbils without the actual SP gene present?  Any other ideas?   AG