TL;DR
Pick the higher-stat parent for every stat you care about, then push the breeding room's Stimulation as high as you can. That alone wins the inheritance roll most of the time. The big shift in Patch 1.1: you can no longer inbreed siblings to consolidate traits and then scrub the birth defects off with a Mutation room. The Mutation stat no longer rerolls defects, Stimulation no longer rescues inbred lines the way it did, and inbred kittens now keep parent defects far more often. The correct play is to keep bloodlines clean from generation one by refreshing with alley strays, not to inbreed first and repair later.
Overview
Mewgenics hides its breeding system behind cute UI and lets players figure it out the hard way. The math underneath is fixed, 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 wrecks your bloodline with defects, and a three-generation chain you can run with the cats the alley actually hands you. The current formulas come from the community wiki's post-1.1 Breeding page, cross-checked against the original datamined extraction. These are numbers pulled from the game code, not estimates.
Core Build
How stat inheritance works
When two cats breed, each kitten stat is a coin flip between the two parents. The flip leans toward the parent with the higher value, and that lean scales with the room's Stimulation:
P(better parent) = (1 + 0.01 x Stimulation) / (2 + 0.01 x Stimulation)
At 0 Stimulation the bias is exactly 50%. At 50 Stimulation it climbs to 60%. Past that it caps near 67%. No Stimulation value makes the better parent a guarantee, so even in a maxed room one in three kitten stats still lands on the worse side of the pair. Negative Stimulation flips the bias toward the worse value, so keep the room positive.
Here is the lever most players miss: 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 enough Stimulation that you can keep rolling kittens until the math falls your way. The bias is per-stat, so a single kitten can pull STR from one parent and INT from the other.
Bred cats roll base stats in the 3 to 7 range. A base 3 in a combat-critical stat is a dead cat. Treat 5 as your baseline for breeding-grade parents and 7 as the ceiling. For the full per-stat breakdown, see our stat inheritance guide.
How ability inheritance works
Active abilities run on their own formulas. Each parent contributes up to two active abilities, and each one's pass-through chance is 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 reaches 100% at Stimulation 32. That threshold is the single most important number in the breeding system. Below it you are gambling on whether the abilities you bred for actually carry. At 32 or above, every kitten inherits at least one chosen ability from each parent.
The second active ability does not hit 100% until Stimulation 196, which is impractical in normal play. Build for first-ability guarantees and treat second abilities as gravy.
Passive abilities use a third formula:
P(passive ability) = 0.05 + 0.01 x Stimulation
Passives lock in at Stimulation 95. If you can drive a breeding room past 95, you stop bleeding passive abilities between generations. Our ability inheritance guide breaks down which abilities are worth chasing across the line.
What Stimulation does to defects in 1.1
In the old system, Stimulation was purely a quality dial. Patch 1.1 gave it a second job: higher Stimulation now reduces the chance a parent's birth defect passes to the kitten. That is a genuine reason to keep Stimulation high beyond the inheritance bias.
The catch is the link to inbreeding. The defect-suppression benefit shrinks as a kitten's inbreeding rises, offset by roughly twice the inbreeding percentage. Breed two unrelated strays in a high-Stimulation room and that room is actively shielding the litter from inherited defects. Breed siblings in the same room and most of that protection evaporates. Stimulation no longer cleans up an inbred line. It only protects a clean one.
The role of Comfort
Stimulation controls quality. Comfort controls quantity. Higher Comfort means cats breed more often; lower Comfort means they fight more often. Comfort changes how fast you get kittens, not how good they are, so do not trade Stimulation away to chase it.
There is a hard ceiling worth memorizing: every cat past four in a single room subtracts one Comfort. Pack five into a room and you lose a point. Pack six and you lose two. Spread your cats across rooms.
Inbreeding and birth defects
Inbreeding is now a compounding penalty, not a one-time gamble you can reroll away. Three separate things go wrong as the inbreeding coefficient climbs.
First, the inbreeding-disorder roll. When a kitten inherits fewer than two disorders from its parents, the game checks for a fresh one:
P(inbreeding disorder) = 0.02 + 0.4 x clamp(inbreeding_coefficient - 0.2, 0, 1)
Below 0.2 inbreeding this sits at the 2% floor. At sibling-level relatedness (coefficient 0.5) it climbs to roughly 14%. At parent-child relatedness (coefficient 1.0) it reaches about 34%.
Second, inbred kittens now inherit their parents' existing birth defects at a much higher rate, on top of that fresh-disorder roll. A defect in the line is far stickier than it used to be. Our birth defects guide covers which defects are survivable and which end a run.
Third, a body-part replacement roll fires once inbreeding passes 5%, with a gain chance around 1.5 x inbreeding. Below 90% inbreeding it swaps one symmetric body part; above 90% it swaps two asymmetric parts, which can push a single kitten to twelve defects and mutations instead of the usual ten.
Stacked together, these mean a sibling pairing is not a 14% gamble you can clean up. It is three penalties at once, and the Mutation room can no longer undo any of them. Keep every breeding pair below 0.2 inbreeding and the disorder roll stays pinned at its 2% floor.
Closeness values to memorize:
| Relationship | Inbreeding coefficient | Verdict |
|---|---|---|
| Parent-child | 0.5 | Avoid |
| Full siblings | 0.5 | Avoid |
| Half-siblings | 0.25 | Avoid unless you have no other option |
| First cousins | 0.125 | Acceptable, sits below the threshold |
| Second cousins | 0.063 | Safe |
Stray cats from the alley are always 0% inbred, which makes them your reset button. The wiki bands match this: under 10% reads as Not Inbred, 10 to 25% as Slightly, 25 to 50% as Moderately, 50 to 80% as Highly, and over 80% as Extremely.
Progression
A three-generation chain you can actually run
This is the pattern to build around. It starts in a low-Stimulation room and ends in one you genuinely want to breed in, and it never asks you to breed back into the immediate family.
Generation 1 (Stimulation 10 to 20). Pull two strays from the alley. Take the highest stat totals you can find, ignore class collars for now, and pair them. Goal: two kittens with at least one stat above 5 and no obvious defects. The inbreeding coefficient sits near zero because you started from unrelated cats. This generation is scaffolding. You are building parents for the cats you actually want, not meta cats themselves. Expect shaky ability inheritance here; the first-ability formula only sits at 45 to 70% per parent in this range.
Generation 2 (Stimulation 25 to 32). Take your best Gen 1 kitten and pair it with a fresh stray that carries an ability you want in the line. A retired cat with a premium ability works just as well at this stage. The point of crossing Stimulation 32 is that the chosen ability becomes guaranteed to carry from one parent into every kitten. Do not pair the two Gen 1 siblings together; they already sit at 0.5 inbreeding, which puts the disorder roll at 14% before the new 1.1 penalties even stack on. Bring in fresh blood instead.
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, again from outside the family. At Stimulation 50 the better-parent bias hits 60% and first-ability inheritance is locked at 100%. Class the result by its stat spread: high CON goes item:tank, high DEX and LCK goes item:hunter, high INT and CHA goes item:cleric or item:mage. Once a Gen 3 keeper is on the field, the stable starts producing replacements faster than your runs can lose them. The class tier list tells you which collars are worth the breeding effort.
Alternatives
Two other approaches show up in community play.
Effect-mutation fishing runs the Mutation stat high to chase body-part mutations with real combat effects, like Extra Leg for movement or item:head-exposed-brain for stacking INT. In 1.1 this is gated: until the Mutation stat clears 10, the room only rolls simple +2 or -1 stat bumps. Effect-bearing mutations gradually open up above 10, so this is a heavy, dedicated investment rather than a free side benefit. And it no longer doubles as inbreeding insurance. The Mutation stat will not reroll an existing defect, so fishing for mutations and cleaning up inbreeding are two unrelated jobs now.
Retired-cat injection skips normal pacing and cycles premium retired cats through the breeding pool to lift their abilities one at a time, then retires the donor. It is strong early and tapers once you already hold most of the abilities you want, and each donor eats a slot a permanent breeder could occupy.
Whichever path you take, the core rule holds: the kitten you want is decided by the parents you pick and the Stimulation in the room they breed in. One 1.1 change you will hear at the breeding screen: a parent's voice now carries to kittens 75% of the time, down from 98%, so litters sound more varied than they used to. Everything else is decoration.