As a joke…

The distinctive glissando had been created quite by happenstance during rehearsals:

As a joke on Gershwin…. [Ross] Gorman [Whiteman’s virtuoso clarinetist] played the opening measure with a noticeable glissando, ‘stretching’ the notes out and adding what he considered a jazzy, humorous touch to the passage. Reacting favorably to Gorman’s whimsy, Gershwin asked him to perform the opening measure that way…. and to add as much of a ‘wail‘ as possible. Source: From Wikipedia, the free encyclopedia