And if they can, can only two of the genes be expressed in a single individual?
There are many genes that are allelic with each other, but yes only 2 can be expressed at the same time. For example: if you have a pairing of Vanilla Fire x Highway all of the offspring will be either : Vanilla or Fire AND Gavel or Yellowbelly.
As Kristen notes, there are many examples of allelic complexes:
- BluEL complex (Bamboo, Butter, Daddy, Honey, Mojave, Mystic, Phantom, Russo, Special…)
- BlkPastel complex (BlkPastel, Cinny, HRA, Huffman, Jolt, Razor…)
- Spider complex (Blackhead, Champagne, HGW, Spider…)
- YB complex (Asphalt, Gravel, Spark, Specter, YB…)
You can only have two of any allele in an animal because of the nature of genetic inheritance - one gene from mom, one gene from dad. So if mom is a Platty and Dad is a Crystal then you can get Daddy/Mojave, Lesser/Mojave, Daddy/Special, and Lesser/Special but you cannot get a DaddyCrystal (Daddy/Mojave/Special) or a PlattyCrystal (Daddy/Lesser/Mojave/Special)
Here’s Matts current list for ball pythons - Allelic Morphs aka Complexes
Think of it as a big long table at a restaurant with planned seating, where couples must sit opposite each other.
This layout doesn’t allow for polyamorous (more than 2) partners to sit together.
So if genes entering the restaurant are YB, Asphalt and Gravel (in the same complex) they will need to play rock, paper scissors to see who is going home.