Personally I have bred silver to silver before I knew it could go wrong and most of the litter were poleys with 2 silvers and if I remember rightly one of the silvers was eaten by the rest, was he defective or something else? I know lads who still breed them together and he only gets small litters of 2 or 3 so I wonder how many are born defective and eaten by the jill or something similar? I'm not entirely clued up on the science behind it but if you Google it you'll find out more info.