White head white tail ringnecks

[TroyTillo]

Hi,
Ive heard that its not a good idea to breed a pair of whwt ringnecks its better to breed a split whwt to a whwt ringneck as you get better size and fertaltiy. Breeding a whwt to a whwt will give you smaller birds and fertaltiy problems. Is this true?? and where or how did the whwt ringneck mutation come from??