Genetic variation

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Genetic variation is considerable between individuals, most of it due to random segregation of chromosomes during meiosis, but some acquired later in life, for instance, in cancer.

The terminology used to describe changes is confusing.[1] The term mutation, as a scientific term, is neutral and simply describes any change in a gene, including changes that have no identifiable functional consequences. The term has taken on lay connotations due to its association with inherited disease, cancer, radiation and science fiction horror stories.[2][3] The term polymorphism is another term describing genetic variantion. While less encumbered by public perception, at least some versions of the definition state that a variant must be present in >1% of the population to be termed a polymorphism.

Nomenclature

May be described by the actual nucleotide change, e.g. 123A>T means that the 123rd nucleotide is a T (thymine) instead of wildtype A (adenosine). Previously, the numbering was based from the 5' end of the mRNA sequence, but more recently, the recommendation is to number from the start/initiation ATG codon.

Alternatively, the mutation can be described by its resulting amino acid change, usually using the alphabet abbreviations, e.g.

  • A substitution is denoted as E123G, meaning that the wildtype E (glutamic acid) has been mutated to G (glycine); X is used when the resulting codon is a nonsense codon
  • In cystic fibrosis, ΔF508, means deletion (Δ) of the normal F (where F = phenylalanine)
  • Missense mutation: one nucleotide in a codon is changed, leading to a different amino acid being coded for.[4] Some single nucleotide changes result in a codon which still codes for the same amino acid - a synonymous mutation - or for one very similar. This is what one expects from evolution.

References

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