Curiouser and Curiouser

To quote Lewis Carroll in Alice in Wonderland.  This is a blip I have been "saving for a rainy day".  Well today was nearly as wet as yesterday  and I realised if I didn't blip it soon it wouldn't be there to blip.

So what does the blips show?  It shows a white cyclamen.  It was bought as a white cyclamen and has flowered over many years as a white.  Suddenly it has a single flower with two petals which are half pink.  The division isas if it had been drawn with a ruler.

Before any cynic asks I can assure you there has been no PS work done to make it pink.  Frankly I don't have the skill to have done that even if I had needed/wanted to.  So the question is, can anybody give me an understandable scientific reason for how/why this has happened?