A quick primer on the genetics of Rhia’s people.
Cassy: Cassy handles the biology,
Sandra: Sandra handles the math,
and our favourite couple gets dragged into it as the case study.
🧬 The Setup
Cassy: “Okay okay.” 🤓🧬✨
Cassy: “So in Rhia’s people, the futa thing comes from a mutation on the X chromosome. We mark it with a little prime symbol: X’ (X-prime!). Regular X is just X. The Y chromosome is totally normal too! X’ is where the magic happens~” 💕
Cassy: “The rule is super simple: anyone with an X’ AND no Y is a futa. Anyone with X’ AND a Y? Totally normal guy! The Y kinda… covers up the X’ in that case!”
Sandra: “Richtig, Kassiopeia. A clean dominance-suppression interaction. Take a look at the full table below:”
Genotype → Phenotype
| Genotype | Phenotype |
|---|---|
| XX | Female ♀ |
| XX’ | Futa ⚥ (heterozygous) |
| X’X’ | Futa ⚥ (homozygous) |
| XY | Male ♂ |
| X’Y | Male ♂ — silent carrier! |
Cassy: “That last row is the spicy one!!” 😳
Cassy: “X’Y guys look and act totally normal, but they pass X’ to ALL their daughters! So a perfectly ordinary-looking dad can have a futa daughter!”
Rhia: “Oh, is that how it works back in our valley, mun? Tha’s tidy, tha’ is.”
Melanie: “Kitty, you’re literally the example.”
Rhia: “Hush, Melly!”
📐 Cross Outcomes (Who Has What Babies)
Cassy: “Now here’s the fun part: What happens when different combos pair up~” 🥰
| Mother | Father | Offspring |
|---|---|---|
| XX | XY | 50% XX, 50% XY |
| XX | X’Y | 50% XX’, 50% XY |
| XX’ | XY | 25% XX, 25% XX’, 25% XY, 25% X’Y |
| XX’ | X’Y | 25% XX’, 25% X’X’, 25% XY, 25% X’Y |
| X’X’ | XY | 50% XX’, 50% X’Y |
| X’X’ | X’Y | 50% X’X’, 50% X’Y |
| XX | XX’ | 50% XX, 50% XX’ (no sons!) |
| XX | X’X’ | 100% XX’ (no sons!) |
| XX’ | XX’ | 25% XX, 50% XX’, 25% X’X’ (no sons!) |
| XX’ | X’X’ | 50% XX’, 50% X’X’ (no sons!) |
| X’X’ | XX’ | 50% XX’, 50% X’X’ (no sons!) |
| X’X’ | X’X’ | 100% X’X’ (no sons!) |
Cassy: “Big detail!!” ☝️😉💡
Cassy: “When a futa is the father, there’s no Y to pass on at all! Those pairings can only produce daughters! Never sons!”
Amy: “So these two couldnae have a lad, then?”
Rhia: “Bechod, like! Bit unfair, innit?”
Cassy: ”(…now I’m super curious whether Siggy might be X’Y…)”
Rhia: “He… he’s not from me place, mun…”
Cassy: “Ohhh… right…” 😳
What It Means for the Population
Sandra: “Now, my favourite part. Suppose every couple produces, on average, the same number of children — regardless of pairing type. Male × female, female × futa, futa × futa, all equally fertile per pair.”
Sandra: “One might naïvely guess: ‘Futa can pair with anyone, so they’ll inevitably become 50% of the population, with 25% female and 25% male.’ This is incorrect, but the actual result is more elegant.”
Implication 1: Female-count and male-count are always equal
At equilibrium, the proportion of plain females always equals the proportion of plain males. Whatever the futa share is, the rest splits exactly down the middle:
- 10% futa → 45% female, 45% male
- 30% futa → 35% female, 35% male
- 50% futa → 25% female, 25% male
It is structural. It falls out of egg-pool and sperm-pool balance.
Implication 2: The futa fraction is a free lore parameter
Without any selection favouring or disfavouring the X’ allele, its frequency simply stays approximately wherever it is, generation after generation. A community at 15% futa stays at 15% futa. A community at 40% futa stays at 40% futa. The genetics has no preferred destination.
Cassy: “Wait wait wait… So the futa percentage in Rhia’s hometown is just… whatever it happened to be when her ancestors settled there??” 🤯
Sandra: “Korrekt. Founder effects, possibly some long-stabilised cultural patterns. The mathematics is agnostic. Rhia’s valley could be 25% futa, 50%, or just 5%. It’s stable either way!”
Rhia: “So I’m as special as I want to be, innit? Proper tidy that is, mun!”
… As special as the author wants you to be… I make the editorial decisions here, after all.
Rhia: “Tha’s what you think, mun!”
Melanie: “It assumes an average number of children per couple…”
Amy: “Ye’re trying tae make a dent in tha’ stat, aren’t ye?”
Cassy: “Well… one couple is not enough to overturn population dynamics!” 😅
Rhia: “We can still try, innit, bach?”
Melanie: “Uhhhm…”
🌸 Side Effects Worth Knowing
- Hidden carriers everywhere. At 25% futa in the population, roughly 14% are X’Y silent carriers. Not rare at all!
- Most futa are heterozygous (XX’). The homozygous X’X’ state is statistically uncommon even in populations where futa are abundant. So most futa carry one X’ and one X.
- A futa-fathered child is never male. Y has to come from somewhere, and futa simply don’t have it.
Sandra: “For canonical purposes: Rhia is almost certainly XX’, not X’X’. The math overwhelmingly favours heterozygous futa at any reasonable population frequency.”
Mio: 💭“O’ course she is, mun! I’m a proper girl, mind!”
Ciri: 💭“We’re not canon yet, dwt!”
Rhia: “Tidy, mun. Whatever that means.”
Amy: “Means ye’re statistically normal, lassie.”
Rhia: “Now that’s a first, innit?”
Melanie: “It sure is…”
Cassy: “Foxgirl genetics is the best!! Thanks for the crash course, Dr. Flößer~” 😘💖
Sandra: “Always a pleasure, Ms. Kanayama. At least someone uses the title.”
The Daily Gyaru — where the genetics is wholesome, the math is rigorous, and the foxgirls are just being foxgirls (sometimes with some huge extra!)