Why is insertion/deletion of three bases less deleterious than insertion/deletion of one or two bases.

Beacuase a codon consists of three bases.

Explanation:

If an insertion or deletion occurs in bases that are not three or multiple of three, the mutation will change the whole sequence of amino acids in translating protein. Earlier the mutation occur the greater will be damage and the result will be totally a new or non-functional protrein. These type of mutations are called as framshift mutations

On the other if the mutation occurs in three bases or multiple of three bases, there would be change in few amino acids (becuase whole codon is indel) and protein may be still functional. That type of mutations are known as In-frame mutations.

Insertion/deletion of 3 base which is a condon will be less fathal because only one amino acid will be missing from the protein of which the gene codes for.

If one codon is deleted it will still be the same but might cause a disease for example in cystic fibrosis one amino acid, 508th Phenly alanine is missing in the CFTR protein. They severity of diseases caused by Insertion or deletion is increased as the number of condon are more.

However, if only one or two is deleted/inserted then all the codons will shift causing them to be in different groups which may form a totally different amino acids as seen in frameshift mutation. This is more dangerous.