"One Gene, One Enzyme" 
Genotype / Phenotype
In Principle: Proteins are the products of genes.
                        Proteins catalyze biochemical reactions.
                        Such reactions produce phenotypes, either directly or indirectly.
                        Different alleles produce different phenotypes
 
Interaction between alleles in diploid organisms is the subject matter of Genetics
  


How do genotypes produce phenotypes?

Beadle & Tatum experiment (1940s) on haploid Neurospora bread mold
    haploid organisms have one allele at each gene locus
    prototroph  wild-type grows on simple medium
    auxotroph mutants cannot grow on simple medium,
                         require supplementation with specific amino acids
          [AKA autotrophs ("self feeder") and heterotrophs ("other feeder"), respectively]

    Hypothesis: "No-growth" phenotype results from a change in the genotype:
                            inability to synthesize amino acid is the result of loss of enzyme activity
                            each mutant corresponds to a defect in a particular enzyme:
                                 "One gene, one enzyme" (Homework)
                            [ Remember: Beadle & Tatum did not know about DNA in 1940 ] 
    Ex.:  arg- mutants cannot grow without arginine, always grow with added arginine
                     particular mutant classes sometimes grow with other amino acids
                        (such as citrulline and/or ornithine)

 Growth response to added amino acids

 

mutants
none
ornithine
citrulline
arginine
arg- 4 -
+
+
+
arg- 2 -
-
+
+
arg- 1 -
-
-
+

      => These other amino acids are involved in the arginine pathway:
               Enzyme defect blocks interconversion of precursors in the pathway.

 Inference of a haploid biosynthetic pathway

arginine pathway w/o genes 

Each mutant class (arg4, arg2, & arg1) affects a different enzyme in arginine biosynthesis
              [cf. methionine metabolism]


Homework:  

1.    In the above discussion,  I have used "mutant" but carefully avoided "mutation":
WHY?

2
.    Critique the following statements:

        "arg- mutants result in defective arginine."
        "arg- mutants are defects of arginine."    
        "arg- mutants are due to absence of the gene for arginine." 
        "arg- mutants are enzymes that block synthesis of arginine."

3. Would you expect to observe an arg mutant class with the following phenotype? Explain.

 Growth response to added amino acids

 

mutant
none
ornithine
citrulline
arginine
arg- X
-
+
-
+



Biochemical Basis of Human Genetic Diseases

In a haploid organism, discovery of DNA shows nature of metabolic  "mutants":

arginine pathway w/ genes

Mutations affect the genes responsible for different enzymes:
     mutate the gene eliminate (or modify) the enzyme

What about diploid organisms?
    Diploid organisms have two alleles at each gene locus: one from each parent

            Interactions between alleles at a locus are the subject matter of genetics

   "Online Mendelian Inheritance in Man" (OMIM) database
            Examples from human biochemical genetics: "Inborn errors of metabolism"
            First three involve disruptions of phenylalanine metabolism

Phenylketonuria (PKU) (Folling 1934) (OMIM citation 261600)
          phenylalanine accumulates in Central Nervous System  mental retardation
          A defect of phenylalanine hydroxylase
          phenyalanine metabolized to phenylpyruvic acid in alternative pathway

         Detection & treatment
             biochemical testing of new-borns: Guthrie Test
             phenylalanine-restricted diet corrects inborn condition  (Euphenics)
                 Maternal PKU results from high fetal [phe] in treated, asymptomatic mothers
                     [ Further information on PKU & related Inborn Errors of Metabolism ]

PKU arises from variation at the Phenylalanine Hydroxylase (PAH) gene locus

    Important: This gene is not a gene "for" PKU: it is a gene "for" PAH

    Diploid humans each have two alleles at this locus
       
Allelic variants produce different levels of PAH activity
       Consider diploid combinations of three (hypothetical) alleles: A, B, & C :
   
Phenotypic consequences of interactions between alleles at the PAH locus

  Genotype PAH Activity [phe]
uM
PKU Phenotype
AA 100%    60      Standard
AB 30%  120      Standard
CC 5%  200 ~ 
300
 
  Hyperphenylalanemia:
 no special diet required
BB 0.3%  600 ~  2400  
Classic PKU:
special diet required

   [Alleles B & C) arise from DNA mutations in the PAH gene]

  PKU is a classic example of a "recessive" genetic disease: What does this mean?

     
AB genotype shows same PKU phenotype as AA  genotype
         that is, A allele shows haplosufficiency: one 'dose' is sufficient to produce standard phenotype
  
         or, expression of A allele "masks" expression of
B allele
                A is therefore "dominant" to B in influencing PKU phenotype
                B is therefore "recessive" to A

        but PAH activity phenotype of AB is intermediate between AABB
                
AB phenotype is closer to BB than AA (0%30% << 100%)
                 B is an "incomplete dominant" to A

        and B produces a higher [phe] phenotype than A:
                Isn't B therefore "dominant" to A ?

    Be careful to distinguish molecular & phenotypic expression (Homework)
   
Homework
Predict the PAH activity, [phe], & PKU phenotypes of the AC and CB genotypes.
  Explain your reasoning.

Would you expect to find a dominant mutation in this pathway?
Why or why not? What might be the nature of such a mutation?

Other molecular disorders of phenylalanine metabolism
    Alkaptonuria (OMIM 203500)
    Oculo-cutaneous Albinism (OMIM 203100)




All text material 2013 by Steven M. Carr