They're still not certain on whether it's two embryo that fuse together, or whether two sperm fertilize one egg. Either way, it does happen in the egg and it does require two 'sets' of something haha. I bet either one could happen. My mosaic was an only child The other egg in the clutch was clear. So that would lead me to believe the two fused together in one egg and left the other empty!go for it, I could just look at him all day! mosaic, now does that happen when development in the eggs they get two sets of something to cause that...my brain is not working I can not think of the words...help me...lol..