Tieflings and other plane-touched are descended from a wide variety of sources. No-one really knows all the answers in a subject like this, especially when so many tieflings are so cagey about their ancestry, and with good reason. However, it is easy to make general statements about the plane-touched. The following list codifies possible fruitful unions on the planes and identifies the offspring if possible. Sorry if this infringes someone's privacy, but the question keeps being asked!

In this list 'human' is used to imply human, half-elf, elf, orc or other mammalian humanoid of similar stature. It is possible for a dwarf or lizard-man to sire a plane-touched offspring, but these events are rare, and the offspring usually gains many characteristics of the fiendish parent.

Female human/Male tanar'ri: Cambion of rank related to his father's status.

Male human/Succubus: Alu-fiend.

Female human/Incubus: Tiefling of strong fiendish characteristics, who will be regarded by baatezu as being a tanar'ri.

Human/Erinyes: Tiefling of strong fiendish nature, regarded by tanar'ri as a baatezu.

Human/Other baatezu: Possibly a tiefling as above.

Human/Yugoloth: Tiefling of strong fiendish nature who will provoke fear in others quite dramatically.

Contrary to what has previously been stated, there are a few human/gehreleth tieflings.

Hordlings are neutral and sterile.

True elementals cannot mate with humans.

Human/Genie: Genasi of appropriate type.

Human/Elemental-kin: Genasi of appropriate type.

Human/Other elemental race: Probably a genasi. Occasionally a true human or no offspring at all.

Human/Aasimon: Aasimar.

Human/Guardinal: Animal-like aasimar. May have whiskers or fur as appropriate.

Human/Eladrin: Elf-like aasimar.

Human/Archon: Aasimar.

Human/Rilmani: A rare mating, but the offspring will be aasimar, tiefling or tenarimar at the DM's option.

Human/Slaad: Are you kidding? That would be the slaad playing with its food! Lizard men or bullywugs might possibly mate with slaadi, if they could run fast. The offspring would be a scaly tiefling, aasimar or tenarimar.

Modrons don't reproduce normally, but a modron wizard might consider a golem its child. See also 'Ylem' and 'Patch' in Uncaged: Faces of Sigil.

Human/Alu-fiend: Tiefling.

Human/Cambion: Tiefling.

Human/Tiefling: 50/50 chance of tiefling or human.

Human/Genasi: 50/50 chance of genasi or human.

Human/Aasimar: 50/50 chance of aasimar or human.

Cambion/Alu-fiend: 50% chance of each parent race.

Tiefling/Cambion or Alu-fiend: 25% chance of Alu-fiend, 25% chance of cambion, 50% chance of tanar'ri-like tiefling.

Tiefling/Tanar'ri: Alu-fiend or Cambion, depending upon the mother's race.

Tiefling/Baatezu: Tiefling of very strong fiendish characteristics, that will be treated as an equal by the baatezu, or at least part of the hierarchy.

Tiefling/Yugoloth: 50% chance of tiefling, 50% chance of nothing.

Tiefling/Tiefling: Tiefling.

Tiefling/Genasi: 25% chance of tiefling, 25% chance of genasi, 50% chance of genniling (see below).

Genasi/Genasi: Only genasi of the same element are interfertile: their offspring are always genasi.

Aasimar/Aasimar: Aasimar.

Aasimar/Aasimon, archon, guardinal, eladrin or rilmani: Aasimar.

Aasimar/Genasi: 25% chance of aasimar, 25% chance of genasi, 50% chance of genniling.

Aasimar/Tiefling or semi-fiend: 50% chance of human, 50% chance of Tenarimar (see below).

Genniling/Genasi: Genasi of same type as genasi parent. Elemental plane-touched of opposed elements are not interfertile.

Genniling/Genniling: Genniling.

Genniling/Anyone else: Human.

Tenarimar/Tiefling: 50% chance of tiefling, 50% chance of tenarimar.

Tenarimar/Aasimar: 50% chance of aasimar, 50% chance of tenarimar.

Tenarimar/Genasi: 25% chance of tiefling, 25% chance of aasimar, 25% chance of genasi, 25% chance of genniling.

Tenarimar/Human: Human.

Tenarimar/Tenarimar: Tenarimar.

The semi-fiends that may be PCs are all those except a baron/marquis cambion. They may be of any non-lawful alignment. They will all be treated as one race here, except where noted. To keep the game balanced, semi-fiends do not gain all the special immunities of tanar'ri. They take half damage from cold-based attacks, lightning, magical fire, poison gas and poisons instead. Genius-level alu-fiends, as described in the Monstrous Compendium, may not be PCs, even though PC alu-fiends may have genius-level intelligence. Alu-fiends have infravision to 240'and a base armour class of 5. Alu-fiends also gain 3 abilities from the tiefling powers table (whichever one you use), re-rolling any immunities to things already specified. Cambion PCs gain only the following special powers: levitate 7 times per day, detect magic once per day per level, and alter self for three turns per level per day. Obviously, PC semi-fiends get to choose what class abilities they have, and do not receive magical items (they are assumed to foreswear these, or have them stolen by their families as they flee the Abyss). As penalty for these powerful skills, semi-fiends, especially cambions, are treated as outcasts and may have to pose as tieflings just to survive. Additionally, they take full damage from iron and silver weapons, and if they handle silver or iron, they must save vs. spell each round or drop whatever it is that they are holding, as it sears their flesh.

Gennilings are the offspring of genasi and tieflings or aasimar. Their genetics, as explained above, make them very rare. A genniling has no special abilities beyond those listed in the tables below, and may be of any alignment. They appear to be humans with pointed ears and a faint hint of some hue about their skin, depending upon their heritage. Playing a genniling is mostly about role-playing one of the planes' most hybrid offspring. They are even more mixed up than tieflings and aasimar, and tend to pretend to be humans. But all the time the suspicion of fiends, celestials and elementals from their past hangs about them. The principal races of planes (elementals, rilmani, baatezu, yugoloths, tanar'ri, guardinals, aasimons, etc.) tend to have an instinctive mistrust of gennilings, seeing then as interlopers from 'the other side'.

Tenarimar are tiefling/aasimar cross-breeds. They may be of any neutral alignment, and get one tiefling special ability, rolled randomly. They are treated with fear by extreme upper and lower planar inhabitants, and with a kind of half-trusting, half-fearful respect by everyone else. They are not quite as rare as gennilings, but almost. They have infravision to 60', and have superior senses of smell derived from such a wide ancestry. However, many tenarimar are prone to fits, as their instincts from both sides of their ancestry clash and they become unable to concentrate, or may suffer a mood swing. Whenever a tenarimar is confronted by a creature of extreme alignment from an outer plane, such as a baatezu, he must save vs. spell or be forced to roll on the following table, unless the planar being shares his own alignment:

The DM should reward the player with XP bonuses if these fits and brainstorms are well-handled.

Tables: These tables sum up the attributes of these new PC races: