I remember reading about a bear classed as a link between brown bears and polar bears.
Post mortem, under a grinning hunters foot.
I remember wondering if he had killed the first and only hybrid that would have bridged the gap for a doomed species. Melodramatic perhaps.
Actually I see there is a wiki!
It seems there is one way gene flow.
https://en.m.wikipedia.org/wiki/Grizzly–polar_bear_hybrid