Leucism:  a general term for the phenotype resulting from defects in pigment cell differentiation. This results in either the entire surface (if all pigment cells fail to develop) or patches of body surface (if only a subset are defective) having a lack of cells capable of making pigment.

Leucism can cause the reduction in all types of pigment, in contrast to albinism, which results in the reduction of melanin production only. More common than a complete absence of pigment cells is localized or incomplete hypopigmentation, resulting in irregular patches of white on an animal that otherwise has normal colouring and patterning. This partial leucism is known as a “pied” or “piebald” effect.

I came across this word today in relation to bird identification. This bird may be a leucistic junco and therefore difficult to identify without the traditional markings.

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