Single gene disorders

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Single gene disorders

DNA genetic code dictates amino acid identity and order

DNA sequence variation in a gene can change the protein produced by the gene

  • allele - one of two or more alternative forms of a gene
  • genotype - genetic constitution of an organism
  • phenotype - observable properties of an organism produced by combination of genotype and environment
  • mutation - alteration in DNA or chromosome sequence or structure, source of alleles and genetic variation

Mutations can be classified according to the type and effect of molecular change

  • a single base substitution is called a point mutation
  • these can have different effects depending on what they are, and where they are in a gene sequence
  • mutations in non-coding regions of a gene can also have important effects

The consequences of point mutations in protein coding regions of genes

  • nonsense mutation - produces premature stop codon
  • missense mutation - single nucleotide is changed, resulting in a different codon and different amino acid

Frameshift mutations

  • insertion or deletion can occur
  • this causes a dramatic change in the protein sequence as all subsequent codons are affected

Mutations affecting metabolic pathways in humans (inborn errors of metabolism)

  • homozygous/homozygote - a diploid organism having the same alleles at one or more genes
  • heterozygous/heterozygote - a diploid organism having different alleles at one or more genes
  • alleles can be dominant (phenotypically expressed in either the homozygous or heterozygous state) or recessive (phenotypically expressed only in the homozygous state)

Phenylketonuria (PKU)

  • recessive mutation of phenylalanine hydroxylase gene
  • homozygotes cannot metabolise Phe
  • accumulating phenylalanine in the blood causes severe mental retardation, slow growth and early death
  • affects 1 in 11,000 to 1 in 15,000
  • routine test of newborn detects high blood Phe
  • Low Phe diet can alleviate the effects of the mutation

Molecular basis of dominance/recessiveness

  • tyrosinase catalyses the synthesis of melanin from tyrosine
  • in oculocutaneous albinism the normal 'wild type' allele of the tyrosinase gene is dominant over the mutant allele
  • heterozygotes with one normal allele make normal tyrosinase and can…


Helen Fogg


I think a 'nonsense' mutation produces a codon that does not code for any of the amino acids :) otherwise, good notes :)

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