Most American bison in the world are privately owned, and therefore the ranchers can breed them with cattle if they want. In fact, you can increase your odds of getting a white beefalo, or creatures of any other color, if you mix them with the appropriate domestic cattle. However, conservation herds such as the ones in Yellowstone and here in Evanston, Wyoming, are meant to preserve some of the bison purity, so they're not going to actively hybridize them. You can often see the Bear River...