The first “genetically pure” bison produced from a cleansed and transplanted embryo was born in June, officials at the Bronx Zoo announced today. Now the zoo can expand its bison herd with only the purest samples of prairie cows.

Yellowstone National Park has two of the last remaining genetically pure wild bison herds in the nation, which used to roam the continent until humans almost wiped them out by the early 20th century. While most current bison have some cattle genes — a result of interbreeding so western ranchers could make hardier cattle – Yellowstone’s American bison are genetically uninterrupted. But they’re difficult to take out of the park for herd-restoration efforts, because they’re susceptible to catching and spreading a wide range of diseases.