frameshift mutation

A frameshift mutation happens when an insertion or deletion changes the number of bases by something other than a multiple of three, throwing off how the genetic message is read. Picture a sentence read three letters at a time: THE BIG RED DOG. Delete one letter near the start and the grouping slides, so the rest becomes gibberish.

The genetic code is read in non-overlapping triplets called codons, each specifying one amino acid. Adding or removing bases in a number not divisible by three shifts this reading frame, so the ribosome misreads every codon downstream of the change. The resulting protein has an entirely wrong amino acid sequence past the mutation point.

Frameshifts are usually severe because they often introduce a premature stop codon by chance soon after the shift, truncating the protein, and because the garbled sequence rarely folds or functions correctly. This is why even a single inserted or deleted base can be far more damaging than a substitution at the same spot.