No this is not a joke, inter species breading is legitimate and has occured numerous times in the past
Examples of this include:
lions and tigers breeding = liger
or donkey and zebra = zedonk
But in these cases the new born is sterile and connot then reproduce to form an entirely new species from combining the species.
TO answer the question of a polar bear and a grizzly bear making a cub - you get a Pizzly bear or Grolar Bear.
A german zoo ( Osnabrück) currently has two Pizzly or Grolar bears on display for tourists and scientists to view and study.
Fascinatingly these bears are not the only Pizzly bears in the wild. Weird bears have started appearing in northern Canada where the two species have been seen in the same habitats together. These bears have had DNA checks on their fur and found some of these Pizzly Bears are second generation.
THIS MEANS A NEW SPECIES IS BEING CREATED RIGHT IN FRONT OF OUR EYES - evolution in the procees.
Reference
http://www.abc.net.au/news/2016-05-19/grolar-pizzly-bears-climate-change-creating-hybrids/7428354
Impossible animals are becoming a possibility.
Its said only possible because polar bears are a relatively new species of bear, that only broke off in the last few thousand years. So the genetics are not so diverse and can still cooperate and produce fertile life.