Leucism: Notes for UPSC

Leucism is a partial loss of pigmentation seen in animals which result in white or pale skin, feathers, hair, scales, or cuticles.

Leucism is a concept that is part of the General Science portion of the IAS Syllabus.

In this article, we shall learn what is leucism by definition and what impact does it leave on animal tissue. IAS Exam aspirants must carefully analyse the information given below as questions based on the same may be asked in the upcoming Civil Services Exam.

Definition of Leucism

Leucism is a term for the physical composition that results from a flaw in pigment cell differentiation. This causes the entire surface or patches of body surface (if only a subset are defective) having a lack of cells that can make any pigment.

All pigment cell-types draw their genetic data from the same precursor cell-type. Hence, leucism can cause a reduction in all types of pigment. This is in contrast to albinism, which leucism is mistaken for. Albinism results in the reduction of melanin production only, though the melanocyte is still present. Thus in species that have other pigment cell-types, for example, fishes that have pigments causing yellow skin the albino variant of these fish species will not be entirely white but instead display a pale yellow colour.

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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, and the ratio of white to normal-colored skin can vary considerably not only between generations, but between different offspring from the same parents, and even between members of the same litter. This is notable in horses, cows, cats, dogs, the urban crow, and the ball python but is also found in many other species.

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How is Leucism different from Albinism?

A further difference between albinism and leucism is in eye colour. Due to the lack of melanin production in both the retinal pigmented epithelium (RPE) and iris, those affected by albinism typically have red eyes due to the underlying blood vessels showing through. In contrast, most leucistic animals have normally coloured eyes. Instead, an outpouching of the neural tube generates the optic cup that, in turn, forms the retina. As these cells are from an independent developmental origin, they are typically unaffected by the genetic cause of leucism.

It can also cause patchy colouration. But the eyes are not affected in this condition. Leucism is different from albinism in that it is caused by a decrease in various types of pigments and not only melanin. It is caused because of defects in the differentiation of pigment cells and/or migration from the neural crest to feathers, hair, or skin during development. 

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