Stat Inheritance and Genetics Deep Dive

Mewgenics presents itself as a roguelike about cats fighting monsters, but underneath that surface is one of the most detailed genetics simulations in any game Edmund McMillen has ever made. Every kitten born in your house carries forward a blended legacy of its parents' base stats, abilities, mutations, and even hidden recessive genes. Understanding this system at a mechanical level is the difference between accidentally improving your gene pool and deliberately engineering it. This guide goes beyond the basics: it breaks down the math behind stat inheritance, explains what Stimulation actually does to the probability curve, covers inbreeding coefficients and their real consequences, and lays out a multi-generation strategy for building a dynasty of elite cats.

The Stat Inheritance Formula

For each of the seven core stats (Strength, Dexterity, Constitution, Intelligence, Speed, Charisma, and Luck), a kitten inherits the value from exactly one parent. There is no averaging. No blending. The game simply picks which parent's value wins for each stat independently. The critical rule is that only base stats can be inherited. Anything added by gear, collars, injuries, buffs, or mutations applied after birth is ignored entirely. If your prized warrior has STR 5 base but STR 9 thanks to a powerful item, your kittens see STR 5 and nothing more.

Base stats in Mewgenics have a natural range of 3 through 7. A kitten cannot inherit a base stat higher than 7 through breeding alone. However, kittens can be born with stats above 7 if a positive mutation applies to them at the moment of birth. This distinction matters a great deal for long-term breeding strategy: if you want a cat with STR 9, you need either a lucky birth mutation or a cat who received it from a mutation event, not simply two STR 7 parents.

A useful heuristic from experienced players: overall combat power follows a roughly multiplicative formula. A cat with STR 6 and CON 6 (36 effective power) outperforms one with STR 7 and CON 4 (28 effective power) for melee-focused builds. This means balanced breeding targets often beat chasing a single stat ceiling.

What Stimulation Actually Does (The Math)

Stimulation is the single most important room stat for breeding. It determines whether a kitten inherits the higher or lower of each parent's stats. The exact probability formula documented from the game's data is: (1 + 0.01 x Stimulation) / (2 + 0.01 x Stimulation). At 0 Stimulation you have a 50% chance of picking the higher stat (a coin flip). At 100 Stimulation that rises to roughly 67%. At 200 Stimulation it approaches 75%.

This formula has important practical implications. Stimulation never guarantees the better stat for any individual roll. Even at high Stimulation you will occasionally get unlucky kittens who inherited the worse stat across the board. But across seven stats, the odds compound: at 0 Stimulation the chance all seven rolls favor the better parent is 0.78% (1/128). At 100 Stimulation it rises to roughly 6%. You need multiple breeding attempts and selection pressure to consistently improve a line.

Stimulation also governs ability inheritance with documented thresholds. At 32 Stimulation and above, inheriting an active ability is guaranteed. At 95 Stimulation passive ability inheritance is guaranteed. At 100 Stimulation the game biases selection toward parents who have class abilities, not random Grey abilities. At 196 Stimulation a kitten can inherit two active abilities instead of one. These thresholds make Stimulation furniture purchases the highest-priority early investment in any serious breeding run.

Genetic Floor vs. Ceiling: What Actually Passes Down

Understanding the floor and ceiling of inheritance is critical for setting realistic expectations. The genetic floor is the minimum a kitten can inherit: 3 in each stat, set by the game's base stat minimum. The ceiling through pure breeding is 7. Any score above 7 that appears on a parent was placed there either by a birth mutation or by a post-birth mutation event, and neither of those sources passes through breeding. The kitten will see the underlying base stat of 7, not the inflated total.

Gear-modified stats also do not pass down. A cat wearing a +3 STR collar has its displayed STR raised, but the inheritance system looks at the base stat value the cat was born with. This is partly why experienced players note that stat-focused breeding only matters from a certain point in the run onward. Early on, abilities are far more impactful than whether a kitten has STR 5 versus 6. The advice to 'breed for abilities early, stats later' reflects this hierarchy.

There is one indirect way gear can affect inheritance: if gear allowed a cat to survive and level up, gaining permanent stat increases, those permanent gains are part of the base stat and do pass on. This makes investing in survivability for your breeding stock doubly valuable.

Mutation and Body Shape Inheritance

Body parts and their associated mutations use a separate inheritance system from stats. Each body part has two gene slots, one from the father and one from the mother. Dominant genes override recessive ones: if a cat has even one Dominant gene for a trait like a Triangle Head, it will always express that trait. A recessive trait only shows visually if both inherited copies are recessive (aa). This means cats can carry hidden recessive traits and pass them to offspring without showing them, a classic Mendelian genetics model.

Body parts are inherited from a parent 80% of the time. The remaining 20% results in a random reroll, independent of both parents. When only one parent carries a mutated body part, the Stimulation-based probability formula applies: the same curve that governs stat selection also governs whether the mutated or unmutated version of a part is passed down. High Stimulation therefore improves your odds of propagating beneficial (green) mutations and, crucially, favors inheriting the better version when parents differ.

Body shapes carry innate stat leans that directly affect which classes perform best on a given cat. Triangle heads favor Intelligence, making those cats naturally better Mages and Psychics. Square heads favor Defense, pointing toward Tank roles. Slender body types lean toward Speed and Evasion, while heavier body types favor HP and Defense. When selecting which kittens to keep, body shape should factor into your class planning: a slender cat bred to be a Fighter or Hunter will hit class stat bonuses more readily than a heavy one.

Bred kittens are capped at a maximum of 10 mutations at birth, enforced by a symmetrization mechanic. Negative (red) mutations can appear regardless of Stimulation level; high Stimulation reduces their prevalence and increases the chance of positive (green) ones, but never eliminates the risk entirely.

Ability and Class Inheritance

Classes are not directly inherited in Mewgenics. A kitten does not simply become the same class as its parents. Class assignment happens when a collar is applied. However, ability inheritance from retired parents is the primary way to propagate class-specific power across generations, and this is where the Stimulation thresholds described earlier become critical.

When cats are retired and placed in the breeding room, their trained abilities become eligible for inheritance. The chance to inherit a first active ability is roughly 0.20 + 0.025 x Stimulation. At 32 Stimulation this eclipses 100%, making the first active ability inheritance reliable. A second active ability has a lower formula: 0.02 + 0.005 x Stimulation, which only becomes reliable at very high Stimulation (around 196). Passive ability inheritance follows 0.05 + 0.01 x Stimulation, with guaranteed inheritance at 95 Stimulation.

One important exception: Skill Share cannot be inherited. High Stimulation also biases selection toward class abilities rather than generic Grey abilities when parents have them. This means a retired Mage cat with Meteor Storm has a meaningfully higher chance of passing that specific ability to offspring at Stimulation 100+ compared to lower environments. The practical strategy is to retire your best-performing cats (the ones with the abilities you want to propagate) into your highest-Stimulation room. Cats with the desired abilities in two slots will pass them to kittens more reliably than cats with one slot filled. Run multiple litters, keep only the kittens who inherited the target abilities, and discard the rest.

Inbreeding: Coefficients, Defects, and the Real Cost

Mewgenics implements a real-world inbreeding coefficient system. Every cat has a coefficient value between 0 and 1 based on its ancestry. Breeding between cats with a relatedness closeness of 4 or lower raises this coefficient. Cats with a closeness of 5 or more actually lower it. Stray cats imported from the world always have a coefficient of exactly 0, making them the most reliable genetic reset mechanism.

The birth defect disorder formula is: 0.02 + 0.4 x clamp(inbreeding_coefficient - 0.2, 0, 1). This caps at a 42% chance of defect disorders when the coefficient reaches 1. Below a coefficient of 0.2 the penalty is a flat 2%, essentially negligible. The danger zone begins past 0.2. Additional body part defects trigger when the coefficient exceeds 0.05 (if a random roll beats coefficient x 1.5). At a coefficient above 0.9, defect body parts are applied in two passes, which effectively writes off the kitten.

Specific defects from inbreeding include Fragile Bones (2x damage received), Hemophilia (persistent uncontrolled Bleeding), Conjoined Body (forced stat tradeoffs), and Sterility (the cat can never breed, ending its genetic line entirely). Sterility is the most dangerous outcome because it removes a cat from your breeding pool permanently without immediately killing it — you may invest resources into a cat before discovering it cannot contribute to the next generation.

Consecutive inbreeding generations compound the coefficient across generations. The 'One and Done' strategy many experienced players use is to inbreed once to lock in a desirable recessive trait combination, then immediately breed that offspring with an unrelated stray to pull the coefficient back toward 0. This lets you consolidate genetics without crossing into the serious defect probability range.

Disorder Inheritance

Disorders follow a different inheritance path than mutations or stats. A kitten has a 15% chance to inherit a random disorder from its mother and an independent 15% chance to inherit one from its father. These rolls are completely independent of Stimulation and cannot be influenced by any room furniture. This means even perfectly bred cats in maximally stimulating environments have a 27.75% cumulative chance of inheriting at least one disorder from their parents. If either parent carries multiple disorders, the inherited disorder is chosen randomly from their pool.

The practical implication: be careful about which cats you breed when they carry problematic disorders. Blindness, Confusion, and similar cognitive disorders are especially painful to pass on since they compound in combat. When evaluating whether to retire a cat into your breeding pool, weigh their abilities and stats against the disorder risk they bring. A cat with three serious disorders is actively making your gene pool worse even if their stats are excellent.

The Stray Cat Strategy: Importing Genetic Diversity

Stray cats are the primary tool for managing genetic health. Because they always carry a coefficient of 0, breeding any cat in your pool with a stray immediately dilutes the inbreeding risk in that offspring's line. Beyond coefficient management, strays serve as carriers of high base stats and novel body shapes that may not exist in your current gene pool.

When evaluating a stray, check both its base stats and its body shape. A stray with STR 7 and a body shape that leans toward Speed is a strong addition to a melee-focused bloodline. A stray with CON 7 and a square head is valuable as breeding stock for your Tank line even if their combat performance is mediocre. The Appeal room stat improves the quality of strays that appear, so investing in Appeal furniture has a compounding benefit on your long-term gene pool.

One advanced technique is the 'stud and rotate' approach: identify a stray with exceptional stats in a key area, pair it with multiple cats in your pool, then breed one of the resulting offspring back into the pool while pairing others with additional strays. This spreads the desirable trait without driving up the inbreeding coefficient of your main line.

Selective Breeding Strategy: Compressing Stats Upward

Stat compression describes the process of using multiple generations of selective breeding to push your average base stats toward the 7 ceiling. The process begins by identifying two cats whose strongest stats complement each other. If your Mage has INT 7 and CHA 5 while a stray has INT 5 and CHA 7, breeding them at high Stimulation gives offspring a reasonable chance of inheriting INT 7 and CHA 7. Keep only the kittens who achieved the best stat combination, retire the rest.

Over 2-3 generations this process concentrates high base stats across your active roster. The key discipline is ruthless selection: do not keep kittens out of sentimentality. A kitten with three stats below 5 is a net drag on your breeding program even if its one outstanding stat is 7. The goal is kittens with useful 7 base stat lines in the stats that matter for their intended role.

Because ability inheritance competes with stat compression for your Stimulation room's throughput (you can only produce so many litters per run), most experienced players sequence these goals. Early runs: focus entirely on propagating the key abilities your team needs. Mid-run: once ability lines are stable, shift attention to selecting for high base stats in the classes those abilities live in. Late-run and beyond: breed for the combination of both.

Multi-Generation Planning: Building a Dynasty

A multi-generation breeding plan needs to track three things simultaneously: which ability lines you are propagating, which stat lines you are compressing, and what the inbreeding coefficient of your pool looks like. The most common failure mode is building a tightly inbred line with excellent abilities and then discovering that defect rates are spiking with every new litter.

A practical four-step cycle works well: (1) Breed your two best retired cats in a high-Stimulation room to produce ability-carrying offspring. Keep the best kitten. (2) Breed that kitten with a high-quality stray to reset the coefficient and add genetic diversity. (3) In the next generation, breed the stray-hybrid with another cat from your main line who carries the same target abilities. (4) Evaluate the output: if coefficient is still healthy, repeat. If coefficient is climbing, insert another stray cycle before continuing.

The payoff for sustained multi-generation planning is substantial. By generation 4-6 of a focused breeding program you can realistically have most of your active cats carrying your target class ability, a top-tier passive, and multiple stats at or near 7 in their primary roles. At that point the house effectively produces combat-ready cats on demand rather than hoping for lucky rolls.

Genetic Diversity vs. Focused Bloodlines: The Real Tradeoff

There is a genuine tension between building a diverse gene pool and focusing heavily on a single bloodline. A diverse pool keeps inbreeding coefficients low, maintains flexibility in which abilities and body shapes appear, and hedges against catastrophic defect streaks. A focused bloodline converges faster on your target build but raises defect risk and reduces the variety of what each new generation can produce.

The meta consensus leans toward semi-focused breeding: maintain 2-3 distinct bloodlines in your house simultaneously, each optimized for a different class role, and cross them with strays regularly. This lets you target specific roles without collapsing all genetic diversity into a single increasingly inbred line. When you find an exceptional stray, evaluate which of your lines benefits most from an injection of its stats or body shape rather than adding it to every line indiscriminately.

The deepest expression of Mewgenics' genetics system is that there is no single 'correct' gene pool. The best breeding programs are responsive ones: reading what your current generation produced, identifying what the next run's challenges likely require, and adjusting pairings accordingly. The cats who win your hardest fights are the ones you designed for those fights specifically, not the ones who emerged from a generic 'make everything as good as possible' breeding strategy.