Why are some DNA mutations sometimes not dangerous, and which type of mutations are these likely to be?

The DNA code is degenerate, which means that a single amino acid can be coded for by more than one sequence of three bases. Some mutations, including substitution, may result in the same amino acid being produced, hence no change in the protein structure. Insertion and deletion mutations are usually more dangerous because they cause a frame-shift, that ultimately changes the order of codons in all the downstream DNA. This changes the protein's primary structure, altering hydrogen, ionic, and disulphide bonds that produce the correct folding in the secondary and tertiary structures. Therefore, a faulty protein is produced that can't carry out its specified function. Insertion of a bases that are a multiple of three are less likely to be dangerous because there is not a subsequent frame shift.