Dictionary of Human Evolution and Biology

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Albinism

Refers to any one of several inherited conditions in animals and plants. In humans, albinism is usually caused by an autosomal recessive allele (see OCA1, below) that blocks a step in the production of the pigment melanin by failure to produce an enzyme essential to the process; associated with a lack of the pigment in skin, hair, eyes and/or other tissues (amelanic melanocytes). The most important forms are
(1) albinism, ocular, type I, X-linked (OA1), an X-linked form, in which only the eyes lack pigmentation; females are more severely affected than males. A second form of X-linked OA also exists (OA2).

See Hermansky-Pudlak syndrome.
(2) albinism, type I, oculocutaneous, tyrosinase negative (OCA1), complete or classic albinism, in which absence of melanin affects the eyes, hair, skin, and hearing. Affected individuals also lack stereoscopic vision as a result of misrouting of optic nerve fibers; they are often also cross-eyed and blind. The impaired protein is tyrosinase. Most OCA1 individuals are compound heterozygotes; aka tyrosinase-negative albinism. Affects at least 1 in 20 000 children worldwide.
(3) albinism, type II, oculocutaneous, tyrosinase positive: in OCA2, functional tyrosinase is present; two phenotypes occur, those with and those without freckles. Sequelae include skin cancer and gross visual impairment. Some pigment is present at birth but lost during childhood. Cause is a deletion in the P protein, which encodes a melanosomal membrane protein. Matings between OCA1 and OCA2 individuals produce double heterozygotes that are unaffected (aka P-gene related OCA2, tyrosinasepositive albinism). This form is common among Nigerian Ibos, the Bantu of South Africa, and African-Americans.
(4) albinism, type III, oculocutaneous, tyrosinase positive: in autosomal recessive OCA3, functional tyrosinase is present; affected individuals are less sensitive to sunlight than OCA1 or OCA2. Freckled skin and reddish hair may be present. The defect seems to be a nonsense mutation in tyrosine hydroxylase that reduces but that does not entirely eliminate enzyme activity. There are about a dozen other forms of albinism.

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