One mare was caught in africa in 1958, she has bred with a normal stallion several times, all offspring of this pair was normal coloured. Then the mother was mated by her own son, she gave birth to white foals. So both parents must have the white gene to get white zebras. So the breeding was totally inbred, but the white zebras couldn't reach a high age, mostly they died under the age of 10.
I know, there were some white zebras in the USA also, but I don't know, if they are still alive.