Breeding is the production of offspring. In Crusader Kings III, this also includes intentional strategies that aim to increase the likelihood of passing down certain congenital traits. This often involves marrying characters with any of the desired traits, and purposely disinheriting heirs that did not inherit the desired trait(s).
Trait Inheritance
Congenital traits
The chance of a congenital trait being passed on is the following:
| Has trait | Nobody | Father's parent (any) | Father |
|---|---|---|---|
| Nobody | 0% | 2% | 25% |
| Mother's parent (any) | 2% | 10% | 50% |
| Mother | 25% | 50% | 40% same trait 40% improved trait |
Active
First the chances of acquiring the trait as an active trait is checked as a function of the parents:
- ACTIVE - ACTIVE: 80%
- ACTIVE - INACTIVE: 50%
- ACTIVE - NONE: 25%
- INACTIVE - INACTIVE: 10%
- INACTIVE - NONE: 2%
Inactive
If that check fails the chances of inheriting inactively is calculated:
- ACTIVE - ACTIVE = 150%
- ACTIVE - INACTIVE = 100%
- ACTIVE - NONE = 75%
- INACTIVE - INACTIVE = 50%
- INACTIVE - NONE = 25%
General logic for trait with tiers inheritance
- Start from highest trait
- Calculate active chance. Lower tiers ( including inactive ) count as inactive of the tier. Each level down beyond the first multiplies chance by a define
- If active, use it; DONE
- Otherwise, go down a tier, if inactive pretend 1 more parent has that tier active, otherwise 1 more parent has it inactive.GOTO 2
When trying to inherit a tiered genetic trait, and the other person has one of lower tier, we pretend that means they've got the higher tier one recessively, and reduce the chance of inheritance by 20% per tier further down than 1 the other parent's trait is When going down a tier, add 40%
Inbreeding
Inbreeding is the process of two characters who are at least 4th degree relatives having children together. Such children can be born with negative congenital traits even if neither parent has them. However there is a small chance for the child to gain the 
Pure-Blooded trait instead. The chance to gain inbreeding traits is low in the first generation but each successive generation has a higher chance. Related characters are also harder to use the Seduction Scheme on unless either the target character's Faith accepts it or if the seducer has the Subtle Desire perk.