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Molecular Basis for Relationship between Genotype and Phenotype genotype DNA DNA sequence transcription RNA translation protein function phenotype organism amino acid sequence One Gene/One Enzyme Hypothesis (Beadle and Tatum) Wild-type Neurospora crassa (red bread mold) is prototrophic. Prototrophs are able to grow on minimal medium. Minimal medium contains only inorganic salts, simple carbon source, and water. Auxotrophs require nutritional supplement(s) for growth. One Gene/One Enzyme Hypothesis (Beadle and Tatum) Conidia are mutagenized with X-ray. Possible mutants are crossed to wild-type and individual ascospores are cultured on complete (permissive) medium. Cultures are plated on minimal medium and auxotrophs are isolated. Auxotrophs are tested for nutritional requirement. One Gene/One Enzyme Hypothesis (Beadle and Tatum) Each requirement for a specific nutrient (mutant effect) followed Mendelian pattern of inheritance. Beadle and Tatum deduced that each mutant was deficient for an essential enzyme in a biosynthetic pathway. A_ B_ enzyme A enzyme B compound Y compound X A_ bb enzyme A compound X compound Z compound Y Not All Gene Products are Enzymes! One Gene/One Enzyme One Gene/One Polypeptide (structural proteins, subunits) One Gene/One Product (rRNA, tRNA) Mutants of Arginine Biosynthetic Pathway cit GSA arg orn glu Mutant 1 + 0 + 0 0 Mutant 2 + + + + 0 Mutant 3 + 0 + + 0 Mutant 4 0 0 + 0 0 cit GSA arg orn glu citrulline glutamic semi-aldehyde arginine ornithine glutamic acid Mutants of Arginine Biosynthetic Pathway Order of Compounds in Pathway Determination of Mutant Effects cit GSA arg orn glu citrulline glutamic semi-aldehyde arginine ornithine glutamic acid Molecular Basis for Relationship between Genotype and Phenotype genotype DNA DNA sequence transcription RNA translation protein function phenotype organism amino acid sequence Gene Sites Sensitive to Mutation null if splice junction is affected no effect if mutation is in the middle of the intron or not the branch point