Mewgenics Ability Inheritance: How Class Spell Favoritism Works
Stimulation is the dial that turns a cat from random rolls into a class-themed carry. Here is the math, the per-class targets, and the room conditions you actually need.
Mewgenics rolls inheritance for every spell on every parent during a breeding event. The chance each roll lands depends almost entirely on one number: the breeding room's Stimulation stat. Pull that number up and you flip kittens from "lucky if they get one cantrip" to "guaranteed to inherit both parents' full active loadouts." On top of that, a second mechanic called class spell favoritism biases inheritance toward whichever class a parent is currently wearing. That is what lets you breed a item:tank cat that throws item:mage-fire-blast or a item:mage cat that drops item:cleric-prayer.
The Inheritance Formula
Three formulas govern ability inheritance, and Stimulation is in all of them.
First active spell, per parent, per breeding event:
P(first_active) = 0.20 + 0.025 * Stimulation
Second active spell, per parent:
P(second_active) = 0.02 + 0.005 * Stimulation
Each passive, per parent:
P(passive) = 0.05 + 0.01 * Stimulation
Both parents roll independently, so a kitten can inherit a spell from neither parent, one parent, or both. The thresholds where each roll becomes a guaranteed 100% are the practical numbers to memorize:
| Stimulation | What gets guaranteed |
|---|---|
| 32 | First active spell from both parents |
| 95 | Each passive from both parents |
| 196 | Second active spell from both parents |
Stim 32 is the early-game milestone every breeding pen should hit. Stim 95 takes deliberate furniture investment. Stim 196 is endgame territory and most players never push there.
A kitten at Stim 50 with two fully-loaded parents is expected to inherit roughly 3.5 of 6 abilities. At Stim 100 that climbs to about 4.8 of 6. At Stim 196 every roll is a coin flip with both sides showing heads, and the kitten inherits everything. Source: SciresM's decompiled gist of the breeding code.
For the parallel math on stat rolls (STR, AGI, INT, etc.), the Mewgenics Stat Inheritance Guide walks the same Stimulation curve from the stat side.
Class Spell Favoritism
Beyond the base inheritance rolls, Stimulation triggers a separate favoritism mechanic:
P(class_spell_favoritism) = 0.01 * Stimulation
At Stim 100, the trigger fires every breeding event. At Stim 50, it fires half the time. When it fires, the inheritance rolls bias toward the parent's currently equipped class spells, meaning the kitten is more likely to pull from that parent's class pool than from anything else.
Each class has a native spell set, and that set is what favoritism amplifies. Breed two parents wearing the same collar and the kitten ends up flooded with that class's signature kit.
Mage parents push offspring toward item:mage-magic-missile, Magic Dart, item:mage-fire-blast, item:mage-frost-blast, and the once-per-battle item:mage-forbidden-flame. The Mage pool leans damage and mobility, with item:mage-teleport and item:mage-warp as utility staples.
Cleric parents bias toward item:cleric-prayer, item:cleric-cure-wounds, item:cleric-healing-word, item:cleric-anoint, and the resurrection chain (item:cleric-awaken, item:cleric-revive, item:cleric-born-again). A item:cleric breeding line is the only practical way to flood a kitten with revive options.
Hunter parents push Lobbed Shot, item:hunter-heavy-shot, item:hunter-snipe, item:hunter-bullseye, and the trap kit (item:hunter-bear-trap, item:hunter-spike-trap, item:hunter-charm-trap). Bullseye is the chase target here. It removes miss chance from every ranged attack the item:hunter kitten ever takes.
Fighter parents push item:fighter-berserk, item:fighter-bull-rush, item:fighter-falcon-punch, item:fighter-counter, and the punch family. Useful for melee carries that want STR scaling without the item:fighter class slot lock.
The kitten inherits these spells but is born with no class assignment. It starts as item:collarless, and you re-class it later into whatever fits the inherited kit. A Tank with three Cleric heals on tap plays nothing like one wearing three item:necromancer summons, and both come out of this same favoritism pipeline.
The February 2026 Patch Fix
A separate class-passive favoritism mechanic shipped broken at launch. Before patch 2026/02/20, the system that should have biased passive inheritance toward a parent's equipped class was firing on the wrong condition, and class-specific passives were rolling at near base rates regardless of Stimulation.
The patch corrected the trigger logic. After 2026/02/20, class-passive favoritism now scales the same way active-spell favoritism does, off the same 0.01 * Stim curve. If you are following older guides written during the launch window, treat any claim about class passives being "unaffected by Stim" as outdated. Source: SciresM gist patch note.
This matters most for the item:necromancer, item:cleric, item:tank, and item:jester pools, where the highest-impact passives live. Pre-patch breeding pipelines built around those classes were shipping kittens with active spells but not the passives, and the fix retroactively makes those pipelines work as intended.
Practical Breeding Advice
Three rules cover almost every breeding decision you will make. For the broader picture on heat windows, room types, and pairing logistics, the Mewgenics Breeding Guide is the upstream reference.
Mate at Stimulation 32 or higher. Below 32, the first-active roll is a probability gamble. At 32 it becomes a guarantee from both parents, which means you cannot lose the breeding event to bad luck on the cheapest, most-cited spell slot. Every breeding room exists to clear at least this bar.
Push the room with Stimulation furniture. The cheap path to Stim 32 is a few starter Stimulation pieces in the breeding room. Hitting Stim 95, the passive guarantee threshold, takes a deliberate furniture loadout: high-Stim pieces stacked, low-Comfort or low-Appeal items removed if they leak budget. Stimulation is the only room stat that affects ability inheritance, so a breeding-only room should pour every furniture slot into it and ignore the others. (Comfort affects breeding frequency, not inheritance, so include it only if you are also impatient.)
Retired parents with rare abilities are gold. A cat that rolled an off-class passive (a item:fighter with item:necromancer Resurrect, for example) is the sole genetic source for that ability in your colony unless you breed it before retirement. Do not let those cats age out without producing offspring. Once they are gone, you have to find that spell on a wild cat or in a new starter and re-import it through breeding from scratch.
A fourth rule worth following: Stimulation does not affect disorder inheritance. That is a flat 15% per parent regardless of room stats. Pumping Stim helps your offspring get more spells; it does nothing to filter out the bad genes. Disorder management is a separate breeding problem solved by selective culling, not by furniture.
Worked Examples
Cleric mom + Cleric dad at Stim 80. Both parents have item:cleric-prayer in slot one. The first-active roll fires at 100% per parent (already past the 32 threshold), so Prayer drops on the kitten almost guaranteed. Class-spell favoritism fires at 80% and biases the rolls toward item:cleric spells when it does. For the second-active slot at Stim 80, the per-parent rate is 42% and the "at least one" combined chance is roughly 66%, biased upward by favoritism on the events where it triggers. Expect the kitten to land at minimum a Prayer plus a coin-flip on item:cleric-cure-wounds or item:cleric-healing-word. Re-class the kitten to anything (item:tank, item:fighter, Bard) and it keeps a working heal kit.
Hunter mom + Mage dad at Stim 100. Mixed-class pairings split the favoritism roll. At Stim 100, favoritism fires every event, but it picks one parent's class pool to bias toward. The expected outcome is a kitten with one parent's full first-active loadout (either Lobbed Shot or item:mage-magic-missile) plus a 76.96% chance of one second-active spell. Useful for hybrid carries, but if you want a clean class-themed kit you breed within one class.
Fighter mom + Hunter dad at Stim 200. Both parents fully loaded. At Stim 200 every active roll is guaranteed and item:mage-forbidden-flame-tier rare passives become reachable. The kitten inherits all six ability slots from both parents, then truncates down to its own slot cap. This is the endgame breeding move that produces "perfect" cats.
What's Next
Ability inheritance is half the breeding equation. If you are setting up a breeding pen for the first time, the Mewgenics Best Starter Classes Guide will tell you which class collars to chase first so your high-Stim rooms are pushing the right spell pools into your kittens.