The birds and the bees of Half-breed mortals

Dragons, elves, and humans are sometimes called the "core races." These are the only mortal races that were created by all of the Nine working in tandem. The other races were created by one, two, three, or four deities.
Only the core races can sire half-breeds. Most hybrids are infertile unless the pairing is between the core races. In other words, half elf/half human, half dragon/half human or half dragon/half elf.
An exception to this rule is with mortal-spirit coupling. "Half-spirits" always greatly favor the mortal's traits and are usually capable of having normal children with their base race though the spirit heritage usually fades quickly over successive generations.
Spirits can only conceive children with mortals if the coupling is consensual, even then conception is not guaranteed. Relative to "ordinary" mortals, half-spirit children, and to a lesser extant the legacies of half-spirit children are more disproportionately likely to develop supernatural Merits and Flaws and they are more likely to be favored souls.

It is very common for halfbreeds on Scarterra to downplay one half of their heritage and enthusiastically champion the values and norms of only one side of their parentage. Half-breeds are more likely to try to immerse themselves in both sides of their heritage in areas where race relations are unusually cordial like Mondert where humans and dwarves generally get along very well.
Half-breeds that choose to embrace their outsider status (or are essentially disowned by both parent's societies) often become fervent Lanterns since Zarthus is widely considered the patron and protector of orphans, half-breeds and bastards.

Common hybrids known
half-elves 
half-human half-elves
Elf-gnome hybrids



Half-human hybrids
half-human half-elves (redunant link, I know)
Man-dwarves
Gnome-human hybrids
half-orcs

Type

Natural