1. Relationship between Genotype and Phenotype
Molecular Basis for
Relationship between Genotype and Phenotype
genotype
DNA
DNA sequence
transcription
RNA
translation
amino acid
sequence
protein
function
phenotype
organism

2. 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.

3. 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.

4. 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 X
compound Y
compound Z A_ bb
enzyme A
compound X
compound Y

5. One Gene/One Polypeptide
Not All Gene Products are Enzymes!
One Gene/One Enzyme
One Gene/One Polypeptide
(structural proteins, subunits)
One Gene/One Product
(rRNA, tRNA)

6. Mutants of Arginine Biosynthetic Pathway
cit
GSA
arg
orn
glu + +
Mutant 1 + + + +
Mutant 2 + + +
Mutant 3 +
Mutant 4
cit citrulline
GSA glutamic semi-aldehyde
arg arginine
orn ornithine
glu glutamic acid

7. Mutants of Arginine Biosynthetic Pathway
Order of Compounds in Pathway
Determination of Mutant Effects
cit citrulline
GSA glutamic semi-aldehyde
arg arginine
orn ornithine
glu glutamic acid

8. Relationship between Genotype and Phenotype
Molecular Basis for
Relationship between Genotype and Phenotype
genotype
DNA
DNA sequence
transcription
RNA
translation
amino acid
sequence
protein
function
phenotype
organism

9. 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