Cystic fibrosis.
"The normal CFTR protein product is a chloride channel protein found in membranes of cells that line passageways of the lungs, liver, pancreas, intestines, reproductive tract, and skin." Mutation of three base pairs changes the amino acid sequence in the protein the gene codes for.
"About 70% of mutations observed in CF patients result from deletion of three base pairs in CFTR's nucleotide sequence. This deletion causes loss of the amino acid phenylalanine located at position 508 in the protein" The end result is a transmembrane protein that is more permeable to chloride.
"People who are homozygous for delta F508 mutation tend to have the most severe symptoms of cystic fibrosis due to critical loss of chloride ion transport. This upsets the sodium and chloride ion balance needed to maintain the normal, thin mucus layer that is easily removed by cilia lining the lungs and other organs. The sodium and chloride ion imbalance creates a thick, sticky mucus layer that cannot be removed by cilia and traps bacteria, resulting in chronic infections."
Changing just three base pairs results in a different gene. The "normal" protein is no longer normal. That's now a different gene. Just like if you're not hugging the knee, you're not in New York. Change a small detail and you've changed the structure. The details matter. All ANY gene does is code for a protein. Change the bases, you change the gene and thus the protein. This may have a small effect or a very, very large effect depending on what the specific change is and where it occurs. There is a tremendous amount of data out there. Google isn't bad, but Google scholar, medline, Ovid, and the individual journal websites are a good place to start. To understand genes, gene flow, drift, and genetic variation I once again highly recommend the Berkely site. Google for "berkeley evolution 101 genes".
http://www.ornl.gov/sci/techresource...ome/cftr.shtml