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Beadle and Tatum, early 1940’s: The “one gene, one enzyme” hypothesis Neurospora crassa Amino Acid Structure a Classes of Proteinogenic Amino Acids primary structure: amino acid sequence of a protein Secondary structure: Tertiary and quaternary structure: Hemoglobin A Phenotype depends on how protein function is changed by a mutation null mutation: • no gene function (no gene product or non-functional product) • usually recessive but can be dominant hypomorphic mutation: • reduced gene function ( protein retains part of its activity) • usually recessive but can be dominant hypermorphic mutation: • enhanced gene function (protein functions more efficiently) • extremely rare • usually dominant neomorphic mutation: • novel gene function (proteins has novel properties or is expressed ectopically, at the wrong place or the wrong time) • dominant Phenotype depends on how protein function is changed by a mutation dominant negative mutation: • reduced gene function, because protein interferes with function of wild-type protein (e. g. as partner in a protein complex) • example: in a complex of 4 identical polypeptides, the chances for an “all wild-type complex” are (1/2)4 = 1/16 Phenotype depends on how protein function is changed by a mutation loss-of-function mutations: null, hypomorphic, dominant negative gain-of-function mutations: hypermorphic, neomorphic