means those job duties that are so fundamental to the position that the individual holds or desires that she or he cannot do the job without performing them. A function can be essential” if, among other things, the position exists specifically to perform that function; there are a limited number of other employees who could perform the function; or the function is so specialized that the individual is hired based on his/her ability to perform it. Determination of the essential functions of a position must be done on a case-by-case basis so that it reflects the job as actually performed, and not simply the components of a generic position description.