New mutations - cataclysmbnteam/Cataclysm-BN Wiki
At the moment the system only covers untargeted mutations.
The targeted ones (alpha mutagen etc.) still use the old one, with the only difference being that it doesn't force negative mutations 33% of the time and doesn't support Robust Genetics, which would turn those 33% forced negative mutations into forced positive ones.
You can use the old system by adding the "Old mutations" mod to a world. This can be safely done to already generated worlds. Same for removal of this mod.
The character passively accumulates inactive "mutagenic potential" over time, which is all turned into new mutations the moment character mutates uncontrollably for any reason. The potential is then cleared.
This potential caps at 7.333 potential mutations, which takes 22 days (1 mutation per 3 days).
Flesh of mutants contains mutagenic toxins, which will cause uncontrollable mutations over a period of time if allowed to accumulate sufficiently.
The character loses about 630 kcal of mutant flesh of mutagen per day. If a starved character gorged on mutant flesh, it would take a little more than 5000 kcal to start mutating.
Once the character starts mutating, all the mutagenic potential is immediately turned into new mutations and the character will continue mutating for at least 4 days.
The character will gain mutagenic potential at a faster rate and will gain about 2 new mutations by the end of it, assuming no more mutagenic toxins. Mutagenic toxins will extend the process and may even speed it up to up to over 1 mutation per day.
Overall the mutations picked will trend towards positive, until the character has a set of traits that is "more good than bad".
Picked mutations trend towards a character with +6 trait score overall. For example, a character with just Quick (+4) and Fleet Footed (+2) traits will receive positive and negative traits about equally, while a character with no positive traits and -12 worth of negative ones will be more likely to get Quick over Fleet Footed and very unlikely to get Junkfood Intolerance.