There are 75 million dog stories in America. This is one of them.

It's not about a hero dog, or a movie dog, or a dog show-winning dog, or a rescue dog, or a therapy dog - just a plain old dog of unknown parentage that happens to be mine.

He came from the city animal shelter, where, after being picked up as a stray a year and a half ago, he was weighed in at 50 pounds, judged to be about 6 months old, labeled a "hound mix" and put up for adoption.

At the time, pretty much everything about Ace (that's the name the shelter gave him) was a mystery - from the breeds that are in him to the home that he came from. What little shelter workers did venture to guess, such as their prediction that he would grow to be a "medium-sized" dog, would turn out to be wrong.

Ace has more than doubled in size since then and now tips the scale at nearly 116. Sprawled out, he's as long as my couch. Standing on his hind legs, he's well over 5 feet. For a while, it seemed he would never stop growing. Eventually, once he turned 2, he did.

But the mystery didn't.

The mystery just grew, fueled mostly by the three, four, five people who would stop me on the street every day to ask about Ace's origins.

I couldn't answer their questions, or my own.

Did he run away or was he abandoned? What sort of home did he start out in? Why does he cower when my hand goes above my waist? What accounts for his sweet disposition, his sociability, his gentleness, not to mention that curlicue tail that seems as if it would be more at home on a shorter, more furry dog?

Most baffling was which of the more than 400 breeds of dogs were in him. Nobody knew, though everyone, from novice to expert, seemed willing to offer a theory.

I was assured once, by a woman who owns one, that Ace is predominantly redbone coonhound. Another insisted he is part Catahoula, a breed developed by Native Americans when they bred domesticated wolves with the mastiffs that invading Spanish explorers brought to the New World. I've even been asked if he is a Kangal, the national dog of Turkey.

More often, the guesses are a mix of two or more of the following: German shepherd, Rottweiler, Labrador retriever, golden retriever, Great Dane, Akita, mastiff, malamute, husky, Rhodesian Ridgeback, pit bull and chow.

I found myself more curious about his heritage than I've ever been about my own.

I know that my background is German, Irish and Welsh - I'm a mix myself, albeit of generally beige tribes. But I've never felt the need to delve into those cultures or proclaim one as my primary heritage, maybe because I'm not 100 percent anything, or even 50 percent anything.

Like my dog, I'm just another mutt, popped out of the melting pot with no particular pedigree - and that's no problem.

In America today, while one can still take pride in one's heritage, or the components thereof, the whole notion of purity, with a few exceptions, has gone out the window.

Society is getting mixed up enough - race-wise, ethnicity-wise - that those things don't seem to matter as much as they once did. Eugenics and fencing in the gene pool are viewed by many as so politically incorrect - and scientifically stupid - they're rarely even discussed.

Except when it comes to dogs.

In the dog world, purity and bloodlines still reign - promulgated by organizations like the American Kennel Club and dog shows like Westminster in New York, where purebreds strut their stuff as an announcer tells us about the various breeds and what they "should" look like.