Remember when I joked about Sigma vs Alpha male tiktoks (on Tumblr)? And how it would be hilarious if we used that for omegaverse theories?
Well, I've been building off my half-humorous addition of Sigma into my Omegaverse worldbuilding/headcanons. I've settled on Theta for the Sigma equivalent for Omega, and settled on some differences and worldbuilding impacts.
(Some standard fare Omegaverse background: Omegas have a sweet scent generally, Omegas/Thetas get pregnant regardless of main sex, Alphas/Sigmas can impregnate regardless of main sex, Alphas have knots - all that is stuff that's fairly standard, but as always, omegaverse can be played around with however a writer likes. These and all stated below are my theorized headcanons.)
Note: I chose Theta because of the medical/neurology connotations of the term - the brainwave state of daydreaming, calm, creativity, etc. It just seemed to have a nice connotation that would be stereotypically associated with 'maternal energies'.

Alpha: can mark Omegas and Thetas, can't be marked. Can impregnate. Can mark as many Omegas and Thetas as they want. Can't hide their secondary gender without perfumes/medication/chemical aids, loses control during ruts. (Optional: Have 2 unique notes in their scent)
Sigma: can mark Omegas and Thetas, can be marked by Thetas. Can impregnate. Can better control their pheromone output innately, remain coherent during ruts. A stealth Alpha, can present as a Beta and will often not realize they are a Sigma unless tested. Monogamous, once marked are restricted by the same rules as marked Omegas/Thetas. (Optional: Have 2 unique notes in their scent)
Beta: can't mark or be marked. Neutral. (Optional: Can sense pheromones, only have 1 unique note in their scent)
Theta: can mark Sigmas (and potentially Omegas), can be marked by Alphas and Sigmas. Can get pregnant. Can better control their pheromone output innately, remain coherent during heats. A stealth Omega, can present as a Beta and will often not realize they are a Theta unless tested. Monogamous, once marked are restricted by the same rules as marked Omegas/Sigmas. (Optional: Have 2 unique notes in their scent)
Omega: can't mark, can be marked by Alphas and Sigmas. Can get pregnant. Can't hide their secondary gender without perfumes/medication/chemical aids, loses control during heats. Monogamous, once marked are restricted by the same rules as marked Thetas/Sigmas.

Beta is recessive to Alpha and Omega genes, but dominant to Theta and Sigma. Theta and Sigma genes have incomplete dominance when paired together, as do Alpha and Omega genes when paired together. Sigma and Omega also have incomplete dominance when paired together, as do Theta and Alpha genes when paired together.
Beta allele will be both 'B/b' depending on if its the more dominant or more recessive gene to the one next to it. Alpha allele will be 'A'. Omega allele will be 'O'. Theta allele will be 't' and Sigma allele will be 's'.
Beta-presenting genotypes are: bb, AO*, st*, Bt, Bs.
Alpha-presenting genotypes are: Ab, AA, As
Omega-presenting genotypes are: Ob, OO, Ot
Sigma-presenting genotypes are: SS, At*
Theta-presenting genotypes are: TT, Os*
The ones marked with an asterick/* are situations of the incomplete dominance. Sigma and Theta genes and Alpha and Omega genes cancel the other out, so to speak, and thus form Beta presenting individuals. But when an Alpha gene and a Theta gene get paired up, the individual will be Sigma presenting. And when an Omega gene and Sigma gene get paired up, the individual will be Theta presenting.

All of these 4 types can be recessive, but this is more akin to intersex in my theories, so sometimes a genetic thing and sometimes an environmental effect on the fetus during development thing. An Omega/Alpha/Sigma/Theta with only one secondary sex chromosome (genotypes of solo: O, A, s, t) will be recessive and all the side effects that go along with that in common Omegaverse lore - decreased or no fertility, weaker pheromones, less debilitating ruts/heats, etc. Or it can be due to hormonal fluctuations (a huge amount or not enough) in their birth parent that could lead to them being born as a recessive variant of whatever secondary sex they are.

Just as certain blood types are more common in different ethnicities in the real world, I theorize Thetas and Sigmas might be more prevalent in some populations and more rare in other populations. Some ethnicities might have more Thetas than Omegas and/or more Sigmas than Alphas, and some populations may be the reverse. Getting into the spread of genes in humans, some human populations in this potential Omegaverse could, a ton of generations ago, had a gene mutation from the Alpha gene into the Sigma gene and the Omega gene into the Theta gene, and then diverged in such a way that their Omega and Alpha genes faded from their population until it was reintroduced by a population that never had that gene mutation occur.

This is the lore my omegaverse AU fics take place in, might add more posts as I develop more lore.