A frame shift mutation is a genetic mutation that is caused by the usertion or deletion of a specific number of nucleotides that shifts
the reading frame of the sequence.
The insertion or deletion of how many nucleotides would cause a frame shift mutation?

The number of nucleotides that would cause a frame shift mutation can be 1, 2 or 4, a number not divisible by three.

Explanation:

The mRNA contains a specific sequence of nucleotides, which are read in sequences of three -triplets or codons- to synthesize a specific sequence of aminoacids. For this to occur properly there is a reading frame, which during translation organizes the sequence of nucleotides that are translated into an aminoacid, as well as the termination sequence.

An error in the reading frame -frame shift mutation- occurs when there is a number of nucleotides in the sequence that cannot be divided by three inserted or deleted from the normal sequence.

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Frame shift mutation example link

b - the respond is innate and varies over time.

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