If the mutation changes the sequence of nucleotides in the DNA, the genotype will change. This change in genotype will give rise to different sequence of amino acids in the polypeptide formed after translation. This different protein may be non-functional or reduced in function, which may ultimately change the phenotype as well.
However, it is important to note that the phenotype may or may not change depending on the type of mutation. If the mutation causes a totally different protein to be synthesized during translation, the phenotype will be greatly affected as well. If the mutation only changes one or two amino acids in the final polypeptide, there might be no visible chanhlge in the phenotype.
