The "white" colouration is a mutation in the pigments of the fur. It is a recessive gene (I think) which means that if 2 tan coloured lions breed, then it is rare, but possible for white lions to be produced.
However, when two white lions are bred together, then the chances of white cubs are drastically improved.
Therefore, the "legend" of 100 years refers to the rareity of white lions in the wild.
(apologies if any of that is wrong, typed it in a hurry and not entirely sure of a white lion's genetics)
I'm sure it's the same, the same thing with white tigers. However ironically the white lions at WMSP are the only pure bred subspecies in the UK, even though they're inbred into oblivion and mutants
