Question

Why is it very unlikely for a frameshift mutation to be a silent mutation?

Answer 1

A frameshift mutation completely changes the genetic code from the point of the mutation, so the protein made as a result of the mutation would have the incorrect structure and would not function as it should.

Answer 2

When a frameshift mutation occurs, the string of DNA will be glued together. When this DNA is going to be part of a process, such as a transcription, whatever is causing the process is unaware of the mutation actually having happened.