Original sentence: In the early 1930s, Berlin Zoo Director Lutz Heck and Heinz Heck of the Munich Zoo began a program that by the 1960s produced the Heck horse, which is a crossbred of Koniks with Przewalski horses, Gotland Ponies, and Icelandic horses.These efforts are sometimes colloquially called "breeding back," though recreating an extinct subspecies is not genetically possible with current technology
Edited sentence: In the early 1930s, Berlin Zoo Director Lutz Heck and Heinz Heck of the Munich Zoo began a program that by the 1960s produced the Heck horse, which is crossbred of Koniks with Przewalski horses, Gotland Ponies, and Icelandic horses