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Questions and Answers
All mutations that provide genetic variation are harmful.
All mutations that provide genetic variation are harmful.
False
Sickle cell disease is caused by a deletion of a codon in the betaglobin gene.
Sickle cell disease is caused by a deletion of a codon in the betaglobin gene.
False
Mutations in the coding sequence always lead to a clinical disorder.
Mutations in the coding sequence always lead to a clinical disorder.
False
Insertions always disrupt the reading frame.
Insertions always disrupt the reading frame.
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Deletions can cause a disruption in the reading frame.
Deletions can cause a disruption in the reading frame.
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Duplications always result in a deletion of a gene.
Duplications always result in a deletion of a gene.
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Mutations can occur spontaneously through errors in DNA replication and repair.
Mutations can occur spontaneously through errors in DNA replication and repair.
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All mutations are dynamic and unstable.
All mutations are dynamic and unstable.
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Heterozygous sickle cell carriers are not resistant to falciparum malaria.
Heterozygous sickle cell carriers are not resistant to falciparum malaria.
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Only chromosomal alterations can cause genetic variations.
Only chromosomal alterations can cause genetic variations.
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Study Notes
Genetic Variations
- Genetic variations can be classified into two types: polymorphism (frequency > 1%) and mutation (does not affect the population)
- Mutations can be heritable alterations or changes in genetic material that can be harmful or beneficial
Trinucleotide Repeat Expansion
- Identified as the mutational basis for single gene neurological disorders such as:
- Fragile X syndrome
- Huntington's disease
- Myotonic dystrophy
- Spinocerebellar ataxia
- Friedreich ataxia
- Classification:
- Polyglutamine (or PolyQ) diseases: repeated codon is CAG, which codes for glutamine (Q) resulting in a polyglutamine tract
- Non-polyglutamine diseases: repeated codons do not code for glutamine
Fragile X Syndrome
- Most common single cause of inherited mental retardation
- Located on the long arm of the X chromosome - "CGG" in the FMR1 gene
- Repeat number and phenotype effect:
- ≤ 45: unaffected individuals
- 50 to 199: no adverse effect on phenotype
- Pre-mutation: over 200
- Full mutation: fragile X syndrome, silences gene expression and causes the clinical syndrome
Substitutions
- Single base pair substitutions
- Can be synonymous or silent mutations
- Non-synonymous mutations:
- Mis-sense mutations: alter the encoded protein and result in abnormal function
- Nonsense mutations: generate a stop codon, resulting in premature termination of translation of the peptide chain
Dynamic or Unstable Mutations
- Triplet repeats: a normal feature in a genome
- Can be unstable and expand, leading to adverse phenotypic effects
Mutations
- Can be heritable alterations or changes in genetic material
- Usually harmful, but can be beneficial in evolution
- Can arise spontaneously through errors in DNA replication and repair
- Examples:
- Sickle cell disease: a single base substitution (6th codon of the beta-globin gene from GAG to GTG)
- Cystic fibrosis: over 700 mutations have been described, with ΔF508 accounting for approximately 70% of all cases in northern Europeans
- Duchenne Muscular Dystrophy (DMD): mutations include deletion, duplication, and point mutations
Insertions, Deletions, and Duplications
- Fixed and stable mutations
- Insertions: addition of one or more nucleotides into a gene, can interrupt the reading frame
- Deletions: loss of one or more nucleotides, can disrupt the reading frame
- Duplications: doubling of the same nucleotides, can cause disorders such as DMD and Charcot-Marie-Tooth disease
Reading Frame
- Important for maintaining the correct sequence of amino acids in a protein
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Description
Learn about genetic variations, including polymorphisms and mutations, and their role in single gene neurological disorders such as Fragile X syndrome and Huntington's disease.
