Over the past 20 years, this is the sort of conversation TV critics have had with network executives and show-running executive producer with alarming frequency:

Critic: "So, why is it that your cast is exclusively white?"

Executive and/or showrunner: "Well, we chose the best people for the parts. They just happened to all be white.

"We'll try to do better in the future."

And then nothing happens.

Which makes me want to sit these people down and make them watch Fox's "Glee." Executive producer Ryan Murphy and his team — who cast their show mostly with TV unknowns — didn't seem to have any trouble finding a diverse cast.

It's not just that the show is so integrated, it's also so effortless. With the exception of one episode in which (mostly) evil cheerleading coach Sue Sylvester (Jane Lynch) tried to divide the glee club along racial lines for her own purposes, the color-blind casting has not tried to deliver a big message.

And that episode, "Throwdown," effectively satirized how others try to use race to divide Americans. And did so without being heavy-handed.

The "Glee" cast (and characters) include African-Americans, Hispanics, Asian-Americans, Jews, gays and whites. There's no attempt to downplay who any of them are, but, again, there's no attempt to beat us over the head with a message.

Which makes the message that much more effective.

This week's episode (Wednesday, 8 p.m., Fox/Ch. 13) carries that message not one but two steps further.

Since the show debuted, it has featured a character who's in a wheelchair. But, other than suffering from the cruelty of some of the "cool" kids, we haven't really seen how Artie (Kevin McHale) has to struggle more than kids who can walk.

That changes in the episode "Wheels." Not only do we get an insight into Artie, but so do the other kids in the glee club. Mr. Schuester (Matthew Morrison) assigns each of them to roll a mile in Artie's shoes by navigating in their own wheelchairs.

And things take an unexpected turn when Sue is ordered to hold open tryouts for a spot on the cheerleading squad — and one of the hopefuls has Down syndrome.

THERE HAS BEEN SOME CRITICISM of "Glee" because Kevin McHale is only playing a kid in a wheelchair. He actually has full use of his legs.

While that criticism is understandable, it's also misplaced. Should a straight actor be disqualified from playing a gay character? Should a Christian actor be disqualified from playing a Jew?

There is, however, a certain irony in the fact that, while many of the other actors who perform in the production numbers have no dancing background, McHale does.

"So for me being ... in a wheelchair and watching everybody else dance is kind of torture," he said.

"But, no, I absolutely love playing somebody in a wheelchair. I've learned so much from it. And I just appreciate the fact that I get to get up after every take."

e-mail: pierce@desnews.com