The following list of races can interbreed with any of the others on the list:
Human
San Elf
Usil Elf
Fir Elf
ALL other races can ONLY breed with members of the same race. Also, Dragon's Gate DOES NOT allow for mixed breeds or mixed abilities. This means female Humans can only have Human babies, female San Elves can only have San Elf babies, and female Fir Elves can only have Fir Elf babies, despite the combination of parents.