Double dwarf gene. Fatal. One dwarf gene creats true dwarfs - the small rabbits that this is all about, no dwarf gene makes a false dwarf - bigger but still small since the breed is small, two dwarf genes are deadly and manifest as peanuts.
If you breed two true dwarfs, you'll get 25% peanuts on average (25% false, 50% true), they live for some days, or even weeks, but eventually die. Breeding a false and a true dwarf avoids that issue but still produces the same number of true dwarfs.
The dwarf gene essentially is a deformaty causing mutation, but it works as long as there is only one, and it makes those cute rabbits.
About rabbits having problems delivering - they need a lot of calcium at that time. It is used up to build the bones of the kits, but it is also needed in sufficient levels to keep the muscles working during delivery. Many give the doe a Tums (fruit flavoured is said to is well accepted) when they are about to kindle, I grind up egg shells to fine powder and put some on her pellets.
