How did the insertion of a nucleotide in a DNA sequence result in a phenotypic change in an organism

if the nucleotide insertion occurs within the coding region of a gene (frameshift mutation), then it will change the open reading frame (ORF) of the resulting protein and thus also will alter the phenotype of the individual  

Explanation:

A mutation is any change in the DNA sequence of the genome of a cell/organism. A point mutation is a genetic mutation where a single nucleotide is inserted, deleted or changed from the original DNA sequence. The insertion of a single nucleotide in the coding region of a gene is called frameshift mutation since this nucleotide base shifts the way the DNA sequence is read. A frameshift mutation changes the meaning of the codons that follow the inserted nucleotide base and thereby protein function is generally lost, thereby also affecting the resulting phenotype associated with this protein.

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