Functional change:

When a new protein is created, representing a change
in function of the gene. Changes in function caused by mutations can be
either gains or losses. A gain-of-function mutation creates an entirely new
trait or phenotype. Sometimes, the new trait is harmless, like a new eye
color. In other cases, the gain is decidedly harmful and usually autosomal
dominant (flip to Chapter 12 for more on autosomal dominant traits)
because the gene is producing a new protein that actually does something
(the gain-of-function part). Even though there’s only one copy of the new
allele, its effect is noticeable and thus considered dominant over the original,
unmutated allele.