"The science behind the phenomenon of variegated leaves is fascinating, even though we don't always apprecaite it. Plants that are made up of two genetically different kinds of cells are known as chimeras. One curiosity is B. chlorosticta, which has stunningly beautiful dark green leaves with a light green margin and spots. It has the same structural variegation as all the other begonias, so why isn’t it just green and silver like them? According to research by a team from Taiwan and published in the journal Annals of Botany, the answer lies in the red pigment in the lower epidermis of the leaves of this species."
See also original paper at http://www.le.ac.uk/biology/phh4/public/Begonia_1065.pdf