1) No real change in a the phenotype
The genetic code of an organism is very long, and not all segments correspond to a phenotype. Often, a mutation in an organism will result in no noticeable change in any phenotype, meaning there will be no visible effect of the mutation on the organism.
2) A small change in a phenotype.
Somtimes a mutation will render a very small change in the phenotype of an organism. One example is a certain type of birthmark, which is caused by a single mutation.
3) A significant change in the phenotype.
A mutation can also lead to significant changes in the phenotype of the organism. One example is a mutation that causes bacteria, or other organisms, to become resistant to viruses or antibiotics. This mutation causes a huge change in the organism's survivability (fitness), and thus will cause it to be passed on often to new organisms.
