Overview
Mewgenics buries its breeding system behind cute UI and lets players figure it out the hard way. The actual math is fixed and datamined, and once you know the formulas you stop gambling on kittens and start engineering them.
This guide covers how the seven core stats pass from parents to kittens, why the Stimulation room stat dictates almost everything, how to read the inbreeding coefficient before it mauls your bloodline with birth defects, and the first three generations of a breeding chain you can run with the cats the alley actually gives you.
All formulas below come from SciresM's datamined extraction of the game code. Numbers are not estimates.
Core Build
How stat inheritance works
When two cats breed, each kitten stat is decided by a coin flip between the two parents. The flip is biased toward the parent with the higher stat value, and the bias scales with the room's Stimulation:
P(better parent) = (1 + 0.01 x Stimulation) / (2 + 0.01 x Stimulation)
At 0 Stimulation, that bias is exactly 50%. At 50 Stimulation it climbs to 60%. At 100 Stimulation it caps near 67%. There is no Stimulation value that makes the better parent a guarantee, so even at maxed rooms one in three kitten stats lands on the worse side of the pair.
The implication: if one parent has 7 STR and the other has 3 STR, you do not need both parents over 5. You need one parent at 7 and a Stimulation value high enough that you can keep rolling kittens until the math falls your way.
Bred cats roll base stats in the 3 to 7 range. A base 3 in a combat-critical stat is what the Steam community meta guide calls "a dead cat." Treat 5 as the baseline for breeding-grade parents and 7 as the ceiling.
How ability inheritance works
Active abilities follow a separate formula. Each parent contributes up to two active abilities, and the probability of each ability passing through is also tied to Stimulation:
P(first active ability) = 0.20 + 0.025 x Stimulation
P(second active ability) = 0.02 + 0.005 x Stimulation
The first active ability hits 100% at Stimulation 32. That number is the single most important threshold in the entire breeding system. Below 32, you are gambling on whether the abilities you bred for actually carry. At or above 32, every kitten inherits at least one chosen ability from each parent.
The second active ability does not reach 100% until Stimulation 196, which is impractical in normal play. In practice, you build for first-ability guarantees and accept that second abilities are gravy.
Passive abilities use yet another formula:
P(passive ability) = 0.05 + 0.01 x Stimulation
Passives hit 100% at Stimulation 95. If you can push a breeding room to 95+, you stop losing passive abilities between generations.
The role of Comfort
Stimulation controls quality. Comfort controls quantity. The breeding attempt formula multiplies compatibility by sqrt(1 + 0.1 x Comfort), which scales sub-linearly. Going from 0 Comfort to 10 Comfort raises the breeding attempt rate by about 41%. Going from 10 to 50 only doubles it again.
There is a hard ceiling on Comfort: every cat past four in a single room subtracts one. Pack five cats into a single room and you lose a Comfort point. Pack six in and you lose two. Spread cats across rooms.
Selecting parent pairs
Two rules:
- Pick the higher stat in every slot you care about. Inheritance bias is per-stat, not per-cat, so a kitten can pull STR from one parent and INT from the other. You only need each parent to be the high-roller for the stats they bring to the pair.
- Check the inbreeding coefficient. This is the sneaky one. Birth defect chance follows:
P(birth defect) = 0.02 + 0.4 x clamp(inbreeding_coefficient - 0.2, 0, 1)
Below 0.2 inbreeding, defect chance sits at the 2% floor. At sibling-level relatedness (coefficient 0.5), it jumps to 14%. At parent-child relatedness (coefficient 1.0), it hits 34%. Pair up siblings often enough and roughly one in seven kittens rolls a defect like Brittle Bones or Trembling Paws.
The fix: track lineage. The community standard is to refresh your bloodline with stray cats from the alley every few generations, and to never breed back into the immediate family unless you are chasing a specific recessive trait and willing to eat the defect risk.
Progression
First three generations of a recommended chain
This is the pattern the Steam community meta breeding guide builds around. It assumes a low-Stimulation starter room and ends with a room you actually want to breed in.
Generation 1 (Stimulation 10 to 20): Pull two strays from the alley. Pick the highest stat totals you can find, ignore class collars for now, and pair them. Goal: produce two kittens with at least one stat above 5 and no obvious defects. Inbreeding coefficient should be near zero because you started from unrelated strays.
This generation is throwaway. You are not building meta cats, you are building parents for the cats you actually want. Expect ability inheritance to be unreliable at this Stimulation range. The first-ability formula sits at 45 to 70% per parent, and second abilities are essentially noise.
Generation 2 (Stimulation 25 to 32): Take the best Gen 1 kitten and pair it with a fresh stray that has a desired ability you want in the bloodline. The Steam guide recommends pulling in a retired cat at this stage if a retired cat with a premium ability is available. Once Stimulation crosses 32, that ability is guaranteed to inherit from one parent into every kitten.
Goal: produce a kitten with the target ability locked in plus stats high enough to function as a Gen 3 parent. Watch the inbreeding coefficient. A Gen 1 sibling pair already sits at 0.5, which puts birth defects at 14%. Don't.
Generation 3 (Stimulation 32 or higher, ideally 50+): Now you have a parent worth keeping. Pair it with a complementary cat that brings the missing stat or ability. At Stimulation 50, the better-parent stat bias hits 60% and the first-ability inheritance is locked at 100%.
Goal: produce your first "main run" cat. Class it according to its stat distribution. A high CON kitten goes Tank. A high DEX/LCK kitten goes Hunter. A high INT/CHA kitten goes entity:cleric or Mage.
Once Gen 3 is on the field, the stable starts producing replacements faster than runs can lose them.
Avoiding birth defects
The single rule: keep every breeding pair below 0.2 inbreeding coefficient. The math sits at the 2% floor under that threshold, which is the lowest defect chance the formula allows.
Closeness values to memorize:
- Parent-child: coefficient 0.5 (avoid)
- Full siblings: coefficient 0.5 (avoid)
- Half-siblings: coefficient 0.25 (avoid unless you have no other option)
- First cousins: coefficient 0.125 (acceptable, sits below the threshold)
- Second cousins: coefficient 0.063 (safe)
Alternatives
The chain above assumes you want a clean, optimized bloodline. Two alternative approaches show up in community discussion:
The mutation-fishing approach keeps a Mutation-rated room high so kittens roll random body part changes more often. This is high variance. You can land Extra Leg (flat movement bonus) or entity:head-exposed-brain (stacking INT per fight), or you can land entity:cataracts and Trembling Paws on the same kitten. Players running this approach treat mutations as a long-term bet across many generations rather than a single roll.
The retired cat injection approach ignores normal generation pacing and just keeps cycling premium retired cats into the breeding pool to grab their abilities one at a time, then retires the donor. This works in the early game but stops scaling once you have most of the abilities you want, and it costs you the slot a permanent breeder would otherwise occupy.
Whichever path you pick, the fundamental rule stays the same: the kitten you want is decided by the parents you pick and the Stimulation in the room they breed in. Everything else is decoration.
For deeper detail on the underlying formulas, the wiki's Breeding Genetics page documents the full set of equations including compatibility, fertility, and body part inheritance.