By the 1880s the wild bison of the plains had been reduced to a few hundred animals. Almost every bison alive now descends from a small number of private herds rescued in that window.
Several of those ranchers deliberately bred bison with domestic cattle, hoping for hardy beef animals. The crosses were largely a commercial failure, but hybrid females were fertile, and their descendants were folded back into bison herds. Genetic surveys since the 1990s have found cattle mitochondrial DNA or nuclear markers in the large majority of tested herds, usually at low proportions.
A handful of populations, most famously in Yellowstone National Park, were long treated as free of detectable cattle introgression and used as a reference for conservation breeding. More recent and more sensitive analyses have reported low-level introgression even there, which is an active argument rather than a settled result. What such small fractions mean for bison health and appearance remains largely unmeasured.