Apple seeks explanation!

The photos below depict two apples from the same tree, my Welsh semi-sweet cooker. Left is a typical fruit, mostly red but with some green showing; right, something quite bizarre: a green apple with a solid red quadrant sector. Not knowing anything about botany, I cannot imagine what the explanation might be.
If you told me the RH one was genetically modified, I could well believe it!

Top 3/4 view
From above
Both apples upside down.