Note: An article on Chromosomal Pair 33 was found in Twilight Sparkle’s library. This is a condensed, layponies’ version that everyone can understand.
Chromosomes 65 and 66 are an oddity. The only genes that interact with magic, they also have sets of suppressor genes that control whether a pony is a Pegasi, Unicorn, or Earth Pony. They are three spoked when in normal operation, but, as one can only observe them during mitosis, the image below displays them in their doubled, 6 spoked form:
Normally, the suppressor genes, at the “hub”, turn off two of the branches, only allowing one to be expressed (e.g. an Earth Pony’s suppressor genes turn off the Unicorn and Pegasi genes). However, with the correct spells, all suppressor genes can be TURNED OFF, allowing all genes to be expressed. This results in the kind of pony known as an Alicorn.
This is not a naturally occurring configuration, and is highly unstable, meaning that, as of current knowledge, this configuration cannot be inherited. Furthermore, because of the instability, the Alicorn configuration requires a closer association w/ magic, thereby granting Alicorns immense magical power as a side effect.
Note: No further information has been found. The publisher invites comments, in order to discover more about this subject.