Depending on the size of the indel, both indels and substitutions are considered a type of point mutation. However, the most fundamental key.
difference between indels and substitutions is:
1. Large indels can be several thousand base pairs in size.
2. Substitutions can result in an indel, but not vice versa
3. Indels can alter the open reading frame, but substitutions cannot
4. Substitutions change the amino acid sequence of a protein

3. Indels can alter the open reading frame, but substitutions cannot

Explanation:

INDEL is a term used in genetics to represent a mutation caused by insertion of nucleotides or deletion of nucleotides from a sequence. As stated in this question, the insertion or deletion can be a point mutation (involving a single base) as well as substitution mutation.

Insertion mutation adds nucleotide(s) to the sequence, deletion mutation removes nucleotide(s) from the sequence and substitution mutation replaces nucleotide(s). Based on the fact that INDEL mutation will either shorten or lengthen the sequence, it can alter the open reading frame whereas substitution cannot.

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