Johnny,
They do have that dark fruit and deep color in common, that's for sure. I looked up the source information and it is noted that Petite Sirah came from cross-pollenating Syrah with a plant called Peloursin (a Rhone grape vine).
When it comes to that whole cross-pollenation thing, it's funny what end results can be. Cabernet Sauvignon was a result of a cross of Cabernet Franc and Sauvignon Blanc. Madness, if you think about it from a 1 + 1 point of view.
Earthquake makes some nice wines - their Zinfandel and Petite Sirah are nice indeed.
- Jim