Genetics in D&D

I'm a big fan of genetic heritage. D&D has a number of different rules for dealing with heritage, and it's never entirely easy to decide what way is best. Between templates, bloodlines, and feats, how do you choose? A little while back, I drew up some simple guidelines for myself. These are mostly based on the relative LA of each option. I count major, intermediate, and minor bloodlines as +3, +2, and +1, respectively.

But they are only guidelines. If anything in a specific case fails to make utmost sense, adjust it until it does.

---

If one of the parents is an X, use a half-X race, if an available one is appropriate. For example, if an orc breeds with a human, use the half-orc race. But if an orc breeds with an elf, find something else.
Otherwise, use a half-X template. For example, if a fiend breeds with a human, apply the half-fiend template to a human.
If there are no available half-X races or half-X templates available in splatbooks, Dragon magazine, or on the web, take X, chop all its stats in half, chop the other parent's stats in half, and combine them.

If one of the parents is a half-X, use the strongest available X bloodline (from Unearthed Arcana). For example, if a half-vampire breeds with a human, use a human with the major vampire bloodline.
If there is no available bloodline, use X heritage feats. For example, if a half-fiend and a human breed, and you're missing that page of UA, use a human with the fiendish heritage feat and as many or as few feats from that feat tree as you desire.
If there are no such heritage feats, make an X bloodline using the guidelines provided in UA.

If one of the parents has a major X bloodline, use the Xish/ic template. For example, if a human with a major celestial bloodline breeds with a human, apply the Celestial template to a human.
If there is no such template, use the intermediate X bloodline.

If one of the parents has an intermediate X bloodline, use a planetouched race. For example, if one parent has an intermediate fiendish bloodline and the other is a human, use the tiefling race.
If no such race exists, use a minor X bloodline.

If one of the parents has an Xish/ic template, use a planetouched race, if appropriate. For example, if one parent has the celestial template and the other is a human, use the aasimar race. If one parent has the celestial template and the other is a dwarf, use something else.
If no such race exists, use a minor X bloodline.
If no such bloodline exists, use X heritage feats.
If no such feats exist, make a minor X bloodline using the guidelines provided in UA.

If one of the parents has a minor X bloodline, is X planetouched, or has X heritage feats, use X heritage feats.

If one of the parents has X werecreature template, use an X werecreature template.

If one of the parents is a mongrelfolk, use a mongrelfolk. Half-mongrelfolk is always mongrelfolk.

---

In theory, these rules can be applied successfully no matter how bizarre and convoluted a character's ancestry is. Consider a situation where one parent is a nixie and the other is a half-vampire tiefling with draconic heritage feats and an intermediate celestial bloodline:

One parent is pureblood fey, so the offspring will have the half-fey template.
One parent is a half-vampire, so the offspring will have the major vampire bloodline.
One parent is a tiefling, so the offspring will have fiendish heritage feats.
One parent has draconic heritage feats, so the offspring will have draconic heritage feats.
One parent has an intermediate celestial bloodline, so the offspring will be aasimar.

Thus, the offspring will be an aasimar with the half-fey template, the intermediate vampire bloodline, and fiendish and draconic heritage feats.