Why are frameshift mutations likely to be more detrimental than point mutations, in which a single pyrimidine or purine has been substituted?

Frameshift mutations are likely to change more than one amino acid, infact a completely irrelevant protein with altered amino acid sequence is produced.

Explanation:

Frameshift mutation is due to addition or deletion of bases. From that point onwards the reading frame shifts, different set of codons read. Frameshift mutations are likely to change more than one amino acid, infact a completely irrelevant protein with altered amino acid sequence is produced.

Frame shift mutation can lead to thalassemia due to premature chain termination and run-on-polypeptude that are non-functional.