Allele frequency determined by genetic rift
I
you! 
Soooo, more about the spirit genome:
We know spirits are formed from ideas. We have Compassion, Wisdom, Desire, blah...
Basically a spirit is formed when collective ideas condense the essence of the fade into a spritual entity. The spirit genome can be some sort of macromolecule composed of Fade particles that "encodes" the spirit essence.
When elves form physical bodies from spirits, they obtain a physical genome, defining their bodies, AND they retain their spirit genome, defining their spiritual essence. So, Solas is an elf, but he can also carry the essence of Wisdom (or Pride) within him.
Modern elves still have these, but they are dormant. They're passed down randomly from one parent of either gender. (Let's assume behavior similar to mitochondrial DNA, cause otherwise this headcanon becomes too complicated).
Since they are dorman post-veil, spiritual characteristics are not selected for and are susceptible to drift. Which is why the modern elves look like Tranquil to Solas, they have been losing their spirit functional spirit genes.
But our Lavellan - she retains hers by chance. She's a spirit of hope!
I've had too much wine tonight.