What makes a mutation dominant?•Haploinsufficiency.In this case, the amount of product from one gene is not enough to do a complete job. Perhaps the enzyme produced is responsible for a rate limiting step in a reaction pathway. Hereditary hemorrhagic telangiectasia (HHT) is an autosomal dominant vascular dysplasia leading to telangiectases and arteriovenous malformations of skin, mucosa, and viscera. Death by uncontrollable bleeding occasionally occurs. It is caused by mutation in the gene ENG, which codes for the protein endoglin, a transforming growth factor-beta (TGF-beta) binding protein. Perhaps the TGF-beta is unable to exert sufficient effect on cells when only half the normal amount of receptor is present.•Dominant negative effect.The product of the defective gene interferes with the action of the normal allele. This is usually because the protein forms a multimer to be active. One defective component inserted into the multimer can destroy the activity of the whole complex. An example might be Osteogenesis imperfecta, see below
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