The Genetic Defect Albinism
In the past, albinos were usually treated with fear or awe. They were sometimes killed at birth. Albino births were common enough in some groups not to cause any excitement. For example, among the San Blas Indians of Panama, one in approximately 130 births is an albino. In the mid-nineteenth century, albinos were exhibited in sideshows. Whole families were displayed at times and were described as a unique race of might people. They were said to live underground and to come out only at night when the light was dim and would not hurt their eyes.
Albinism is a genetic defect if the integumentary system. Albinism occurs when the body fails to produce melanin. Melanin forms in a special cell called the melanocyte. This cell is found in the skin, in the hair follicle, and in the iris and retina of the eye. There are many steps in converting the amino acid tyrosine to melanin pigment. Two types of melanin form: black-brown eumelanin and red-blood pheomelanin.
Trosiase is the major enzyme involved in the formation of melanin pigment. Tyrosinase is responsible for converting tyrosine to DOPA and on to dopaguinone. The dopaguinone then forms black-brown eumelanin or red-yellow pheomelanin. The tyrosinase enzyme is made by the tyrosinase gene on chromosome 11, and alterations of this gene can produce on type of albinism because the tyrosinase enzyme made by the altered gene does not work correctly.
Two additional enzymes called tyrosinasa-related protein 1 or DHICA oxidase and tyrosinase-related protein 2 or dopachrome tautomerase are important in the formation of eumelanin pigment. The gene for DHICA oxidase in on chromosome 9 and the gene for dopachrome tautomerase in on chromosome 9. Alterations of the DHICA oxidase gene are associated with a loss of function of this enzyme and this causes one type of albinism. An alteration of the gene for dopachrome tautomerase does not produce albinism.
Three other genes make proteins that are also involved in melanin pigment formation and albinism, but the exact role of these proteins remains unknown. These genes are the P gene on chromosome 15, the Hermansky--Pudlak syndrome gene on chromosome 10, and the ocular albinism gene on the X chromosome.
The gene carrying the defect that produces albinism is recessive, which means that both parents must carry this recessive gene in order to produce a child with the condition. When both parents carry the gene (and neither has albinism), there is a one in four chance with each pregnancy that their child will have albinism. The inheritance pattern of ocular albinism is alittle different. This condition is X-linked, meaning that the recessive gene for ocular albinism is located on the X chromosome. X-linked ocular albinism appears just about only in males who inherit the condition from their mothers.
Albinism is a recessive inherited defect in melanin metabolism in which pigment is absent from the hair,...