In the late 1990s DNA profiling determined that Petite Sirah in California was a field blend of Peloursin, some actual Syrah and mainly Durif, which is a crossing of Peloursin and Syrah.
Huh?
Let’s break it down. Petite Sirah is a darkly-colored, fairly tannic grape grown in warm climates like California, South America and Mexico. Sometimes it’s bottled as a single varietal – I especially like the ones from Elyse, Judd’s Hill and Neal Family – these wines are bold, with spicy blue and black fruits. At other times it’s used to beef up red blends.
A field blend, not as common as it once was, is a mixture of different varietals planted in the same vineyard.
Peloursin is an obscure French grape, now found in California and Victoria, Australia.
A crossing is when two varieties within the same species are combined to create a new varietal. Think Pinotage (Pinot Noir x Cinsault) or Müller-Thurgau (Riesling x Sylvaner).
Durif was spread into southeastern France in the 1880s by Dr. Durif and while it was resistant to diseases such as downy mildew, it didn’t really produce any high quality wine and the French authorities weren’t too keen on having it very widely planted. You can now find it in North and South America and several areas of Australia; Rutherglen, Riverina and Riverland.
A Thirsty Spirit 
Regarding: “In the late 1990s DNA profiling determined that Petite Sirah in California was a field blend of Peloursin, some actual Syrah and mainly Durif, which is a crossing of Peloursin and Syrah.”
Much more simple that what you’ve written: Syrah + Peloursin = Durif (aka: Petite Sirah).