TL;DR
Birth defects are a probability roll your breeding choices feed. The inbreeding coefficient between the two parents, written r, is the lever that decides almost everything. Keep r at or below 0.05 and you breed clean. Patch 1.1 raised the stakes: the Mutation stat no longer scrubs defects off a kitten after birth, inbreeding now also amplifies inherited defects, and Stimulation has become a real second lever for cleaning up what a parent already carries. Avoiding defects at breeding time is the whole game now, because there is no longer a back-end fix.
Two Categories: Disorders and Parts
Defects split into two pools that use different formulas and roll independently. Understanding the split is the single best thing you can do before your next litter.
Disorders are negative trait packages: stat penalties, behavioral effects, or battle-start debuffs. item:adhd, item:depression, and item:schizophrenia all live in this pool. A failed disorder roll picks one at random.
Parts are mutated body parts. Some are cosmetic flags like item:bushy-eyebrows or item:blue-spots. Others carry real weight: item:no-eyes makes a kitten permanently Blind, and item:cataracts stack a 5% miss chance per turn. A failed parts roll picks from this pool.
The split matters because the formulas behave nothing alike. The disorder roll only climbs past r = 0.2 and tops out near 42% in the worst engineered cases. The parts roll fires on any inbreeding above r = 0.05 and reaches 75% at sibling closeness. Parts do the heavy damage long before disorders ever wake up, which is why a pairing that looks "only a little" inbred can still hand you a litter of broken bodies.
How Defects Are Triggered
The disorder roll:
P(disorder) = 0.02 + 0.4 * clamp(r - 0.2, 0, 1)
The 2% baseline applies to every breeding event, even between fully unrelated cats. Risk only starts climbing past r = 0.2. At full sibling pairing (r = 0.5), disorder probability is 14%. At r = 1.0, the worst case you can engineer, it caps at 34% per pass.
The parts roll is harsher and triggers far earlier:
trigger: rand() < 1.5 * r, condition: r > 0.05
Any closeness above r = 0.05 opens the parts roll. By r = 0.5 you sit at 75%. By r = 0.667 you hit a guaranteed mutated part.
Both rolls run twice when r > 0.9. Two passes at r = 1.0 produce two mutated parts and a 56% chance of at least one disorder. This multi-pass rule is why parent-child breeding wrecks a lineage so fast.
The breeding UI shows five named tiers: Not Inbred (r <= 0.10), Slightly Inbred (0.10 to 0.25), Moderately Inbred (0.25 to 0.50), Highly Inbred (0.50 to 0.80), and Extremely Inbred (above 0.80). The math runs on the raw coefficient; the labels just tell you which formula bracket you landed in.
What Patch 1.1 Changed
The old strategy let you breed dirty and clean up afterward. That door is closed. Three changes in 1.1 rewrote how defects move through a line, and all three push toward the same conclusion: stop them at conception.
The Mutation stat no longer rerolls existing mutations or birth defects. A kitten born with a defect keeps it. There is no second chance on the back end, so every defect that rides along on a high-r pairing is now permanent.
Inheritance is now coupled to inbreeding. The chance a kitten inherits a parent's existing birth defect is significantly higher in inbred kittens. Breeding a defect-carrying parent into an already-tight line is now doubly punishing: you risk fresh defects from the two rolls above, and you raise the odds of passing down the ones the parent already has.
Stimulation flipped from liability to lever. It used to treat parent defects like positive mutations and increase their inheritance. Now it decreases the chance of inheriting a parent's birth defect, so a high-Stimulation room actively cleans your odds. The catch: Stimulation's benefit shrinks the more inbred the kitten is, so it rescues a careful line, not a reckless one.
The coefficient is still the primary lever for new defects. Stimulation is now the secondary lever for inherited ones. Treat them as a pair.
The Most Impactful Defects
A handful of entries deserve recognition by name. Most are landings you reroll on sight. Two are tools you aim at on purpose.
Primordial Dwarf
The marquee trade. Six stats take a hit in exchange for a large Luck spike, concentrating the payout into a single stat the breeding system cannot reach through normal inheritance. On a Jester collar or any Luck-scaling build, a item:primordial-dwarf kitten is a crit-fishing carry. Players target this defect deliberately.
Savant Syndrome
The mirror. Six stats lose ground for a large Intelligence spike. This is the defect for any spell-heavy collar. A Mage or Cleric carrying item:savant-syndrome leapfrogs generations of stat inheritance work. Same farming logic as Primordial Dwarf.
Down's Syndrome
Trades Intelligence for Constitution and Charisma. The Charisma feeds starting mana while the Intelligence loss starves regen, so the trade pays out in the opening turns and sours if a fight drags. item:downs-syndrome is workable, not build-defining.
Williams Syndrome
Trades Intelligence for a big Charisma stack and a chance to inflict Charm on attack. The Charm-on-hit is the real lever: a frontline cat can lock down single targets without spending mana. That combat utility separates item:williams-syndrome from Down's.
Depression
The worst disorder by a wide margin. Every stat drops, the affected cat opens battle with All Stats Down, and adjacent allies take All Stats Down every turn. A Depression cat poisons its own formation just by sitting on the board. No build redeems it.
Osteogenesis Imperfecta
Brittle bone. The cat opens combat with Bruise, Bleed Thorns, and Thorns stacked on itself, and takes an extra injury when downed. A frontliner with item:osteogenesis-imperfecta folds on the first big hit. A few retaliation builds turn the Thorns outward, though the math rarely beats a reroll.
ADHD
Stats lean positive, with a bonus to Speed and Intelligence. The cost is a decision timer each turn; let it expire and the cat acts on its own. Pause defeats it. ADHD is classed as minor and is genuinely playable, and some players treat the speed bonus as a free upgrade.
Schizophrenia
At end of turn, the cat spawns a short-lived hallucination with Madness and Doomed. It cannot summon minibosses, bosses, or champions. Free board pressure every round, paid for in chaotic targeting. Crowd-control comps welcome it; tight synergy comps suffer.
When Defects Are Good
Two defects flip the math, and only two. Both produce a concentrated stat spike normal inheritance cannot reach, which is exactly why they are worth chasing.
Primordial Dwarf as a Luck carry. On a Jester or a Luck-scaling Thief, the Luck spike crit-fishes every basic attack and triggers Luck-gated abilities at rates the rest of your roster cannot touch. The penalty across the other stats stings, but on a build where Luck is the only stat that matters, the trade clears.
Build: b0a5302302226224Savant Syndrome as an Intelligence spike. Mirror logic. The Intelligence spike on a Mage or Cleric is a spellcaster two breeding generations ahead of schedule, and the penalty rarely bites a backline caster.
Farm the target defect on the lowest r that reliably triggers a disorder roll. At sibling pairing (r = 0.5) the disorder roll lands 14% of the time, so expect roughly seven breedings per success. Here is the part 1.1 forces you to accept: every other defect that lands on those same high-r attempts is now permanent. You cannot scrub it later. Treat failed kittens as discards, not fixer-uppers, and never run the farm hotter than you need to.
How to Avoid Defects
Keep r below 0.05 and the disorder roll sits at its 2% floor with no parts roll at all. That is the clean line, and a closed pool can hold it forever.
The game caps inbreeding accumulation at closeness C = 5, which is third cousins. Once two cats are three generations apart, they breed as if unrelated. So a closed pool runs indefinitely as long as you keep three generations between any mating pair. Lineage tracking is the whole discipline.
The rules that keep a line clean:
- Track every cat's lineage three generations deep. A spreadsheet works fine. The breeding UI tier labels handle quick spot checks.
- Never breed full siblings or parent-child unless you are farming a target defect. The combined defect rate at
r = 0.5runs around 78%, roughly four litters in five. That is an outcome, not a risk. - Half-cousins (
r = 0.0625) are basically free. About 5% parts risk and zero disorder risk, the cheapest way to lock a rare passive that exists in only one family line. - Push Stimulation when a parent carries a defect. A high-Stimulation room now lowers the odds that defect passes down, as long as the pairing itself stays loose.
- Watch the tier label. Anything above Slightly Inbred should be a deliberate call, never a convenience pairing.
For the upstream mechanics of inbreeding coefficients, kinship walks, room conditions, and how the closeness cap interacts with closed-pool breeding, see our Mewgenics Breeding Guide. For how a clean kitten's stats carry into the next generation, our Stat Inheritance Guide covers the math the defect rolls sit on top of.
The heuristic is short enough to keep in your head. Below r = 0.05 you are clean. Above r = 0.5 you are gambling. Above r = 0.9 you are losing the kitten and probably the line. Two defects are tools you aim at on purpose; the rest is what happens when the math gets away from you. Once a clean kitten is in hand, the collar decides what it becomes, and our Mewgenics Class Tier List ranks every collar from S to D so you know what you are holding.