Your children are extremely important, as one of them will usually take over upon your death. To ease succession and subsequent rule, you want to prepare your potential heirs for the throne. One important way of preparing your heir for the throne is by making good decisions even before he has been conceived. As large parts of the game's birth system seem to be hardcoded, more of this installment will be based on personal experience than usual, rather than on the game files themselves. I'll go into how breeding works, and what you can do to affect it.
Basics of Breeding
Depending on your and your spouse's fertility, you're more or less likely to have children. Two months after conception you'll be notified. Approximately seven months later, a child will be born. Essentially everything is decided upon birth, not conception; your child's gender and hereditary traits. Gender is a simple 50-50 split. Hereditary traits however seem to be a bit more complicated.
As the code for hereditary traits seems to be in the .exe itself, how exactly it works is not something I can find out. I can however base it on personal experience. From what I can tell, hereditary traits are based merely on two factors: The hereditary traits of the parents, and how similar their genetic material is.
From the save-files we can know that every single character in the game has a DNA value attributed to them. Further study shows that a character's DNA is based on a combination of their father and mother, with a small degree of mutation. For example the DNA values "dgfhibhieeh" and "jgagiicckmi" combined to create "dgfgibccepi", taking the first, second, third, fifth, sixth and ninth letter from the father, and the rest with the exception of the 10th from the mother. It can further be assumed that negative hereditary traits become more common the more letters of the DNA strings are shared, while positive traits become less common. This is likely graded through some sort of scoring system, as the congenital traits have no value assigned to them beyond their effects. It can be assumed that the worst of the congenital traits only become common when many letters of DNA are shared, while the most positive congenital traits are at the other end of the spectrum.
As every character has eleven letters of DNA, it is reasonable to assume that five are taken from the father, five from the mother, and one is random. From the save-files we can also see that no letter higher than 'p' is used; there are 16 characters in use. Due to this it is unlikely that any random character will have more than one or two characters in common with you, and based on this, one shared character likely has little or no penalty. Thus we can calculate how far away from you in your family tree for only one character to be shared. As five characters are inherited every generation out of 11, we thus get three separations before most characters share only one letter with you. As such I would recommend at least four degrees of separation whenever breeding within your family tree. Thus if you share grandparents (2 degrees of separation each way, for a total of 4) for example, marrying is unlikely to negatively affect any children you might have.
There are several congenital traits. These seem to be inherited about 50% of the time, and it is likely the presence of one prevents its opposites from presenting due to inbreeding. Below I've compiled a list ranked roughly from worst to best.
- Imbecile: -8 to all traits, -10 opinion with all vassals, -30 opinion with anyone of the opposite sex. Opposed to slow, quick, and genius
- Inbred: -5 to all traits, -30% fertility, -1.5 health, -20 opinion with all vassals, -30 opinion with anyone of the opposite sex
- Slow: -3 to all traits. Opposed to imbecile, quick, and genius
- Syphilitic: -1 to all traits -2 health, -20% fertility, -30 opinion with anyone of the opposite sex
- Weak: -1 martial, -1 health, -5% fertility, -10 opinion with all vassals, -10 opinion with anyone of the opposite sex. Opposed to strong
- Hunchback: -1 martial, -5 opinion with all vassals, -30 opinion with anyone of the opposite sex
- Dwarfs: -1 martial, -30 opinion with anyone of the opposite sex
- Ugly: -1 diplomacy, -20 opinion with anyone of the opposite sex. Opposed to fair
- Clubfooted: -1 martial, -10 opinion with anyone of the opposite sex
- Harelip: -1 diplomacy, -10 opinion with anyone of the opposite sex
- Lisp: -1 diplomacy, -5 opinion with anyone of the opposite sex
- Stutter: -1 diplomacy
- Fair: +1 diplomacy, +30 opinion with anyone of the opposite sex. Opposed to ugly
- Strong: +1 diplomacy, +2 martial, +2 health, +10% fertility, +10 opinion with all vassals (+20 with tribal), +10 opinion with anyone of the opposite sex. Opposed to weak
- Quick: +3 to all traits. Opposed to imbecile, slow, and genius
- Genius: +5 to all traits, +5 opinion with all vassals Opposed to imbecile, slow, and quick
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. It might even be worth breaking the 4 degrees of separation rule for Strong or Genius, but I'd recommend still keeping at least two degrees of separation, preferably three.
On the other hand you should avoid negative congenital traits whenever possible, especially the worse ones. With a bit of bad luck they could end up plaguing your family for generations to come. Further, if you somehow end up with one of them it might be a good idea to, if possible, marry someone with a positive trait opposed to it, as that likely decreases the chance of your heir having it.
Make good decisions even before your heir is born, and you'll have an easier time when he takes over. My tips are as follows:
- If you can, avoid marrying within your family tree
- If you still want/need to, maintain at least four degrees of separation when marrying whenever possible. Sharing grandparents or any separation higher than that should be fine
- 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 opposed to it (E.G., Quick as opposed to Slow), it should decrease the chance of your heir getting the negative trait
Written by Meneth