|
Albinism in humans (from the Latin ''albus'', "white"; ''see extended etymology'', also called achromia, achromasia, or achromatosis) is a congenital disorder characterized by the complete or partial absence of pigment in the skin, hair and eyes due to absence or defect of tyrosinase, a copper-containing enzyme involved in the production of melanin. It is the opposite of melanism. Unlike humans, other animals have multiple pigments and for these, albinism is considered to be a hereditary condition characterised by the absence of melanin in particular, in the eyes, skin, hair, scales, feathers or cuticle.〔(【引用サイトリンク】title=Albinism )〕 Albinism results from inheritance of recessive gene alleles and is known to affect all vertebrates, including humans. While an organism with complete absence of melanin is called an albino (,〔("Albino" ) Dictionary.com. Accessed May 11, 2011〕 or )〔("Pronunciation of albino" ) Macmillan Dictionary. Accessed May 11, 2011〕 an organism with only a diminished amount of melanin is described as leucistic or albinoid.〔Tietz, W. A Syndrome of Deaf-Mutism Associated with Albinism Showing Dominant Autosomal Inheritance. Department of Pediatrics, Southern California.〕 Albinism is associated with a number of vision defects, such as photophobia, nystagmus and amblyopia. Lack of skin pigmentation makes for more susceptibility to sunburn and skin cancers. In rare cases such as Chédiak–Higashi syndrome, albinism may be associated with deficiencies in the transportation of melanin granules. This also affects essential granules present in immune cells leading to increased susceptibility to infection. ==Signs and symptoms== In humans, there are two principal types of albinism: oculocutaneous, affecting the eyes, skin and hair, and ocular affecting the eyes only. Most people with oculocutaneous albinism appear white or very pale, as the melanin pigments responsible for brown, black, and some yellow colorations are not present. Ocular albinism results in light blue eyes, and may require genetic testing to diagnose. Because individuals with albinism have skin that entirely lacks the dark pigment melanin, which helps protect the skin from the sun's ultraviolet radiation, their skin can burn more easily from overexposure. The human eye normally produces enough pigment to color the iris blue, green or brown and lend opacity to the eye. In photographs, those with albinism are more likely to demonstrate "red eye," due to the red of retina being visible through the iris. Lack of pigment in the eyes also results in problems with vision, both related and unrelated to photosensitivity. Those afflicted with albinism are generally as healthy as the rest of the population (but see related disorders below), with growth and development occurring as normal, and albinism by itself does not cause mortality,〔 although the lack of pigment blocking ultraviolet radiation increases the risk of melanomas (skin cancers) and other problems. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「albinism」の詳細全文を読む スポンサード リンク
|