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.
Marriage
- 主条目：Marriage
As the positive traits are quite rare, the player is recommended to go out of their way to obtain them. With a little bit of luck they could end up staying in the family for a long time. On the other hand, negative genetic traits should be avoided when possible, or when the advantage cancels out the risk. With a streak of bad luck, bad genetic traits could end up plaguing a dynasty for generations to come. If the player somehow ends 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 the heirs having mediocre attributes and relations penalties because of it.
With that being said, some of the lesser negative traits (such as 
or 
) can be somewhat useful for getting a few more points to use in the 
Ruler Designer, as typically about 85% of the born offspring are unaffected and it is usually possible to ensure that one of them becomes the heir(ess).
Since genetic traits are random, breeding programs frequently require either:
- Feudal elective succession
- Open succession
- Designated heir succession
- Or a willingness to advance children through your line of succession with disqualification or murder.
Even with two 
parents, there is no guarantee that the offspring will be a too, but if the player character raises several children, the odds of at least one of them getting this trait are moderately high (~66% in a scenario with two parents and three sons born).
Pregnancy
A married couple's fertility decides the ability and likelihood of conception. With 
Conclave DLC active, it is recommended to get the ambition 
Groom an Heir, which increases fertility by +20%. Without the Conclave DLC activated, the ambition 
Have a Son/Daughter increases fertility by +25%.
It is also possible for the wife to get 
through events, e.g., if the male character becomes rather angry with the death of his firstborn son, or through 
events.
Two months after conception, the player will be notified and the wife will gain the trait and approximately seven months later, a child will be born.
Note that pregnant women can be incapacitated by a Troubled Pregnancy, triggering a regency if she is a ruler in her own right; the Final Months of a pregnancy will always trigger a regency for female rulers.
Pregnancy chance is also affected by FERTILITY_BASE_MULT = 0.5, and the following relationship multipliers:
| Relationship | Multiplier | Modding |
|---|---|---|
| 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 |
Birth
Essentially, everything is decided at birth and not at conception:
- Child's sex is a simple 50/50% chance
- As for congenital traits, each have an inheritance chance built in, which ranges from +15% to +50%
- There is a small chance of genetic traits randomly showing up at childbirth despite neither parent possessing any trait
- The baby may get the
trait and the mother may die while giving birth - Whether or not a pregnancy results in
, it is decided at conception; all of the twins' traits (including their genders) are decided at birth, as usual.
Note: Some genetic traits can also be acquired later in life, such as getting 
from one of the outcomes of a childhood event.
Inbreeding
Characters whose parents have shared ancestors are at risk of being born with the 
trait, severely limiting their skills, fertility, and lifespan.
The risk is higher when the parents are more closely related (specifically, with more shared ancestors). As such, it is recommended to maintain a high degree of separation when marrying and avoid union between close relatives, unless there is a significant advantage.
The chance of the and 
traits depends on the number of unique ancestors the child has in the past five generations (the first of the five generations being their parents), with a 0% chance of both when all ancestors are unique, rapidly increasing as the number of ancestors shrinks. Missing ancestors (i.e., characters without parents) are assumed to all be unique.
| Number of unique ancestors | Chance of |
Chance of
|
|---|---|---|
| At least 32 | 0-10% | 0-2% |
| 28-31 | 23.25-25.5% | 4.65-5.1% |
| 23-27 | 35-39% | 7-7.8% |
| 18-22 | 80-88% | 16-17.6% |
| 17 or fewer | 100% | 27-36.6% |
Once inbreeding has taken root, it has a 25% chance of being passed on to the affected character's children (assuming this marriage is not also incestuous, in which case it is amplified by the above chances), therefore, it is difficult to uproot.
Inbreeding is also a major concern for Zoroastrians and others who benefit from incestuous marriages. Thus, 
becomes particularly important for them.
Non-congenital lineage traits
Muslims
The 
trait is based on agnatic descent from close relatives of the Prophet Muhammad. If a character marries a woman possessing the trait, her sons will be 
. However, a 's child will not become . Because each of these traits offer a +5 opinion bonus with all Muslims and make it easier to become a Caliph, they are desirable for Muslim rulers.
If the player is not running a dynasty, it is very difficult to breed it into the dynastic line. The player is required to invite a into court and pass the Notable Status of Women law so the player will be able to form a 
, or convert the character to a religion that allows for such a marriage. Next, the character should be married matrilineally to a woman of the player's dynasty. If a male child of this union inherits, the dynasty will have the trait from this point forward.
Bastards
- 主条目：Bastard
Children born outside of marriage or are considered bastards. Initially, they have the 
trait, preventing them from inheriting or passing on the dynasty of their parents. However, a parent can choose to make them a 
, allowing them to inherit, but the -1 
diplomacy attribute penalty remains.
Byzantines
Children born to a ruling Emperor of the Byzantine Empire automatically receives with a +5 vassal opinion bonus and priority in succession.
Children of concubines
- 主条目：Polygamy
Children of concubines get the 
trait. They can inherit normally but incur a -1 diplomacy attribute penalty.
Zoroastrians
A Zoroastrian ruler who fully controls the Persian Empire can earn the 
trait. All of their descendants benefit from the 
trait, regardless of gender.
Castes
- 主条目：Caste
Most characters following Dharmic religions are assigned to one of three castes:
Each caste has a different advantage, but followers of the Hindu religion have -30 opinion penalty toward rulers whose caste does not match their holding type. Children inherit the lower one of the castes their parents have.
List of genetic traits
Positive
| 名称 | 键值 | ID |
|
|
|
|
|
健康 | 出生 % | 继承 % | 好感 | 其他修正 | 杂项 | 花费 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
魅力非凡 | fair | 53 | +1 | 1% | 15% | +30 吸引力好感 | 与  相反
|
16 | ||||||
|
|
天才 | genius | 56 | +5 | +5 | +5 | +5 | +5 | 0.5% | 15% | +5 封臣 | +10 个人实战能力 +30 理性 |
解锁聪明的伏击作战战术。 与  ,  和  相反
|
30 | |
|
|
敏锐 | quick | 57 | +3 | +3 | +3 | +3 | +3 | 1% | 15% | +5 个人实战能力 +15 理性 |
解锁聪明的伏击作战战术。 与 , 和 相反
|
15 | ||
|
强壮 | strong | 61 | +1 | +2 | +1 | 1% | 15% | +10 吸引力好感 +5 封臣 +10 部落 |
+10% 生育能力 +10 个人实战能力 |
与  相反
|
25 |
Negative
| 名称 | 键值 | ID |
|
|
|
|
|
好感 | 出生 % | 继承 % | 好感 | 其他修正 | 杂项 | 花费 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
|
足内翻 | clubfooted | 48 | -1 | 0.5% | 15% | -10 吸引力好感 +5 同一特质好感 |
-5 个人实战能力 | -4 | ||||||
|
侏儒 | dwarf | 55 | -1 | 0.25% | 25% | -30 吸引力好感 +5 同一特质好感 |
-15 个人实战能力 | 如果父母都有 特质，继承几率增加到50%。与  相反
|
-14 | |||||
|
兔唇 | harelip | 49 | -1 | 0.5% | 15% | -10 吸引力好感 +5 同一特质好感 |
-4 | |||||||
|
驼背 | hunchback | 50 | -1 | 0.5% | 15% | -30 吸引力好感 +5 同一特质好感 -5 封臣 |
-19 | |||||||
|
|
痴呆 | imbecile | 59 | -8 | -8 | -8 | -8 | -8 | 0.5% | 15% | -30 吸引力好感 -10 封臣 |
-30 个人实战能力 -30 理性 |
与 , 和 相反
|
-65 | |
|
|
近亲缺陷 | inbred | 60 | -5 | -5 | -5 | -5 | -5 | -1.5 | 25% | -30 吸引力好感 -10 封臣 |
-30% 生育能力 -20 个人实战能力 -20 理性 |
如果父母都有 特质，继承几率增加到50%。
|
-71 | |
|
|
口齿不清 | lisp | 51 | -1 | 0.5% | 15% | +5 同一特质好感 |
解锁军令费解作战战术 | -1 | ||||||
|
|
迟钝 | slow | 58 | -3 | -3 | -3 | -3 | -3 | 1% | 15% | -5 个人实战能力 -15 理性 |
与 , 和 相反
|
-15 | ||
|
|
口吃 | stutter | 52 | -1 | 0.5% | 15% | +5 同一特质好感 | 解锁军令费解作战战术 | -4 | ||||||
|
|
丑陋不堪 | ugly | 54 | -1 | 1% | 10% | -20 吸引力好感 | 与 相反
|
-11 | ||||||
|
|
虚弱 | weak | 62 | -1 | -1 | 1% | 15% | -10 吸引力好感 -5 封臣 -10 部落 |
-5% 生育能力 -10 个人实战能力 |
与 相反
|
-22 | ||||
|
|
巨人症 | giant | 327 | -0.5 | 0.5% | 5% | +5 封臣 +10 部落 -5 吸引力好感 |
-20% 生育能力 +10 个人实战能力 |
与 相反
|
||||||
|
左撇子 | lefthanded | 337 | 0.75% | 25% | +5 同一特质好感 -10 基督教教会 -10 伊斯兰教 |
+15 个人实战能力 |
Strategy
In order to deal with dynasty characters with negative genetic traits, the following actions may be taken into account:
- If descendants are in the line of succession: The player could try to disqualify them from succession
- If descendants are not in the line of succession: It is much easier to just arrange a marriage into other important dynasties, which also has the possibility of introducing the defect into those dynasties' gene pools. In the long term, this "infection" has the potential to erode their power through generations of dysgenic leaders.
- Alternatively, if no options for disinheritance is available, one might attempt to liquidate the unprofitable assets.
Summary
- Avoid marriages among close relatives; ensure that the groom and bride at least do not share any grandparent
- Avoid marriages with spouses bearing negative genetic traits
- Look for candidates of marriage who possess positive genetic traits
- Should the player happens upon a negative genetic traits, it is possible to marry spouses with positive traits opposite in effect to it (e.g., as opposed to ), for it will decrease the chance of heirs receiving a large hit to their stats.