The cattle in fact have a mutated myostatin gene, which usually restricts muscle growth. However, in hyperplasia sufferers, the gene doesn’t function properly. As a result, calves born with the condition have double the amount of muscle fibers and very little body fat.
Hyperplasia isn’t a modern phenomenon, either; the first recorded case dates back to 1808. Since then, the condition has been purposefully bred into Belgian cattle crossbred with U.K. shorthorns. But this “double muscling” is a relatively modern practice that started properly in the 1950s.
The man responsible for this phenomenon was Professor Roger Hanset, who was an employee at an artificial insemination center at the time. Through a method of diluted inbreeding – also called linebreeding – the double muscling mutation was maintained. And nowadays, it’s a constant feature of Belgian Blue cattle.