Mutations and Abnormalities

• Mutation


  A change in the structure or amount of an organism's genetic material
• Mutation


  • Can be a tiny change in DNA structure or a large scale change in chromosome structure or number
  • When a change in genotype produces a change in phenotype, the individual is called a mutant

  
• Types of gene mutations

  • Point mutations (substitution and inversion)
  • Frameshift mutations (insertion and deletion)

  
• Point mutation


  A substitution or inversion of one or more nucleotides, usually resulting in a minor change (one different amino acid)

  
• Frameshift mutation


  An insertion or deletion of one or more nucleotides, resulting in a major change and a non-functional protein

  
• Phenylketonuria is a hereditary disorder caused by a genetic defect which disrupts the metabolic pathway for phenylalanine

  
• In Britain, newborn babies are screened for PKU, and sufferers are put on a diet containing minimum phenylalanine

  
• Cystic fibrosis


  A genetic disorder caused by a mutation in the gene for the glycoprotein that makes mucus

  
• Cystic fibrosis occurs in 1:2500 births in Britain
• Most mutant alleles are recessive, but some are expressed in the first generation due to being dominant or sex-linked
• Mutagenic agents


  • Chemicals like bromouracil, ethidium bromide, peroxides, and mustard gas
  • High-energy radiation and ultraviolet light
• Chromosomal abnormalities can occur due to non-disjunction during meiosis, resulting in gametes with an abnormal chromosome complement

  
• Down's syndrome


  Caused by non-disjunction of chromosome 21, resulting in an extra copy of chromosome 21

  
• Down's syndrome occurs in 80% of cases due to maternal age
• Turner's syndrome


  Caused by a gamete with no sex chromosomes fusing with a normal X gamete, resulting in a female with 44+XO chromosomes
• Klinefelter's syndrome


  Caused by an XX egg fertilised by a normal Y sperm, or a normal X egg fertilised by an XY sperm, resulting in a male with 44+XXY chromosomes
• Chromosomal mutation disorders


  • Cri du Chat syndrome (missing piece of chromosome 5)
  • Klinefelter syndrome (XXY)
  • Turner's syndrome (XO)
  • Patau syndrome (trisomy 13)
  • Edward's syndrome (trisomy 18)