On a rainy summer day near the Mendenhall River in Juneau, a birder has her binoculars trained on an odd-looking bird. It looks like a robin, but it's completely white. She gets a good look, and takes a few pictures. It is a robin, and it's totally white.
Albinism and leucism are two conditions that can cause animals to have white fur, hair, skin or feathers. Albinism affects an entire animal, but leucism can affect an entire animal, or just patches. Animals with partial leucism are referred to as "peid" or "piebald." A leucistic animal will have normal-looking eyes. Leucistic lions are white, but have normal eyes and lips. Animals with albinism have pink or red colored eyes. A close look at the pictures reveals that the robin has red eyes. An animal with albinism has red colored eyes because the retina and iris lack pigment, revealing the underlying blood vessels.
An animal with albinism has melanocytes, the cells that make the skin pigment melanin, but the animal lacks the ability to process an enzyme that produces melanin. Leucistic animals do not have melanocytes at all - so the skin, hair, or feathers in the affected areas can't produce pigment. They have regular eyes because embryonically, eyes develop separately and from different precursor cells than skin does, and the pigment-producing melanocytes in the eyes develop normally.