Breeding is the idea of marrying someone with genetic traits in order to pass those traits on to offspring and heirs. These traits are given at birth, and at any time only a small percentage of the character population in Crusader Kings II will have them. These traits range from great to terrible, and good genetic traits can be a great boon to any ruler. Getting beneficial genetic traits into a dynasty is what breeding is all about.
Main article: Marriage
As the positive traits are quite rare, you should go out of your way to obtain them. With a little bit of luck they could end up staying in your family for a long time.
On the other hand, you should avoid negative genetic traits whenever possible, especially the worst ones. With a streak of bad luck they could end up plaguing your family for generations to come.
If you somehow end up with one of them it might be a good idea to (if possible) marry someone with a positive trait opposite in effect to it, as that will somewhat decrease the chance of your heir having mediocre attributes and relations penalties because of it. With that being said, some of the more minor negative traits (such as clubfooted or lisp ) can be somewhat useful for getting a few more points to use in the Ruler Designer, as typically about 85% of your children are unaffected and it is usually possible to ensure that one of them becomes your heir.
Since genetic traits are random, breeding programs frequently require either:
- Feudal elective succession,
- Open/Designated Heir succession,
- or a willingness to advance children through your line of succession with disqualification or murder.
Even with two genius parents, there is no guarantee that you will get a genius as your first-born son, but if you have several sons, the odds of at least one of them being a genius are moderately high (~66% in this scenario with two genius parents and three sons born).
Couples tend to have two children on average, but traits that increase fertility can make a couple produce far more than this. The steps for the pregnancy are the following:
- Depending on your and your spouse's fertility, you are more or less likely to have children. Note that the ambition Have daughter/son increases fertility by 25%. It is also possible for the female spouse to get pregnant through events, for example if your male character becomes rather angry with the death of his first-born son.
- Two months after conception you will be notified and your spouse will gain the pregnant trait.
- Approximately seven months later, a child will be born.
Pregnancy chance is also affected by FERTILITY_BASE_MULT = 0.5, and the following relationship multipliers:
|Secondary wife or concubine||0.5||SECONDARY_SPOUSE_FERTILITY_MULT|
|Lover (not spouse)||0.5||NOT_SPOUSE_FERTILITY_MULT|
|Spouse and Lover||1.5||MARRIED_LOVERS_FERTILITY_MULT|
Essentially everything is decided upon birth and not at conception:
- child's gender is a simple 50-50 split
- genetic traits, each have an inherit chance built in, which ranges from 15% to 50%
- small chance of genetic traits randomly showing up at birth despite neither parent possessing the trait
- since patch 2.0 the baby may get the sickly trait and the wife may die while giving birth.
- whether or not the pregnancy is with twins is decided at conception, all of the twins' traits (including their genders) are decided at birth as usual.
Note that some genetic traits can also be acquired later in life, such as getting stutter from one of the outcomes of a childhood event.
The exact mechanics behind inbreeding are unknown (likely based on DNA resemblance), but it appears that the more closely the parents are related, the more likely it is that the child will have the inbred trait.
As such it is recommended to maintain a high degree of separation when marrying and whenever possible:
- preferably never marry within the same dynasty, but it may at times be worth it for the sake of claims or traits.
- it is recommended to avoid marrying grandparents or closer.
Once inbreeding has taken root, it has a 25% chance of being passed on to the affected character's children (assuming this marriage isn't also incestuous, in which case it is higher), so it can be difficult to weed out of the dynasty.
Inbreeding is a big concern for Zoroastrians rulers who benefit from incestuous marriages.
The Sayyid and Mirza traits are based on a character's descent from the Prophet Muhammad. Because they offer opinion boni with all Muslims and make it easier to become Caliph, they're desirable for Muslim rulers. If marrying a woman with the Sayyid trait, the sons by her will be Mirza (however the child of a Mirza is not a Mirza).
If you are not starting as a dynasty that is Sayyid, it is very difficult to breed it into your line. You have to get a Sayyid into your court, convert him to Christianity or pass the Notable Status of Women law so he'll matrilineally marry, and then marry him matrilineally to your daughter. Then you inherit to your grandson, who will be of your dynasty and Sayyid.
A Zoroastrian ruler that restores the Persian Empire gains the Saoshyant trait. Similarly, all his descent benefit from the Saoshyant descendant trait.
List of genetic traits
To deal with dynasty characters with negative genetic traits:
- If they are in the line of succession, try to disqualify them from succession.
- If they are not in the line of succession, it is much easier to just marry them into other important dynasties, which also has the possibility of introducing the defect into those dynasties' gene pools. In the long term, this has the potential to erode their power through generations of dysgenic leaders.
- If you can, avoid marrying within your family tree.
- If you still want/need to, avoid marrying grand parents or closer
- Avoid marrying anyone with negative genetic traits
- Do try to marry someone with positive genetic traits
- If you get a negative genetic trait, marry someone with a positive trait opposite in effect to it (e.g., Quick as opposed to Slow ), it will decrease the chance of your heir receiving a large hit to their stats