The mysteries of the color and pattern genetics makes raising Icelandic sheep extremely interesting. The white ewe at left developed a mysterious blue/grey spot on her back after shearing in the fall. I suspected that she carries the grey gene as her recessive, since her sire is a black/grey ram. Her base pattern is white, which covers up her secondary pattern. Usually white sheep are pure white, or exhibit some tan phaeomelanin markings (as seen in her face), but occasionally mysterious markings do appear.
Sure enough, Gwendolyn's first lambing produced a set of twin ewes: one was white and one was grey! She had been bred to a black badgerface ram, so the grey lamb was a result of the recessive grey gene that I suspected she carried. This was the first time I had seen evidence of the underlying pattern showing up on the sheared coat of a white sheep and it was fun to know that my guess was correct. Gwendolyn has gone on to lamb many more times for us and she always produces either white or grey lambs.