Many traits follow a classical dominant / recessive pattern of inheritance, whereby one allele (dominant) is expressed over the other (recessive)

• Homozygous dominant and heterozygous forms will be phenotypically indistinguishable and the recessive allele will only be expressed in the phenotype when in a homozygous state
  

At a cellular level, there is no universal mechanism by which dominant and recessive alleles act – the pattern of expression will be determined by the particular actions of the proteins they code for

• Recessive proteins may be fully expressed in heterozygotes but their phenotypic capacity is masked by the dominant protein
  

• Recessive alleles may have their expression switched off or significantly reduced by regulatory proteins, resulting in less protein being produced
  

• Recessive alleles may code for non-functioning versions of a protein, so that only the dominant protein can exert a phenotypic influence
  

When representing complete dominance, the convention is to capitalise the dominant allele and use a lower case letter for the recessive allele

• An example of this mode of inheritance is mouse coat colour – dark or black coats (BB or Bb) are dominant to lighter coats (bb)
  

Phenylketonuria

Phenylketonuria (PKU) is a recessive genetic condition caused by mutation in a gene on chromosome 12 that codes for the enzyme phenylalanine hydroxylase (PAH)

• The enzyme converts the amino acid phenylalanine (essential) into tyrosine (non-essential) and in cases of phenylketonuria this metabolism is impaired

• The excess phenylalanine is instead converted into phenylketone, resulting in a toxic build in the blood and urine (hence phenylketonuria)
  

Untreated PKU can lead to brain damage and mental retardation, as well as other serious medical problems

• Infants with PKU are normal at birth because the mother is able to break down the phenylalanine during pregnancy
  

• Diagnosis of PKU is therefore made by testing the blood for elevated phenylalanine levels shortly after birth

• Because the condition is recessive, both parents must be either heterozygous carriers of the allele (or have the condition themselves) in order to pass the disease to their offspring

PKU is treated by enforcing a strict diet that restricts the intake of phenylalanine to prevent its build up within the body

• This low-protein diet should include certain types of fruits, grains, vegetables and special formula milk
  

• This diet should be supplemented with a medical formula that contains precise quantities of essential amino acids

• Patients who are diagnosed early and maintain this strict diet can have a normal life span without damaging symptoms

Phenylketonuria