The Dwarf Gene
There are a number of rabbit breeds which rely on the dwarf gene to fit their ideal standard. These include: Netherland Dwarfs, Dwarf Hotots, Holland Lops, Mini Rex, Mini Satins, American Fuzzy Lops, Lionheads, and Jersey Woolies. (There are a few small breeds which do not normally have the dwarf gene, including Polish, Dutch, Brittania Petites, and Florida Whites, but occasionally do because of outcrossing).
The dwarf gene is a incompletely dominant gene, where two copies of the dominant gene is actually lethal. Such double dwarfs are often called peanuts. Peanuts are often stillborn, or will only live a couple of days due to poorly-formed internal organs. Peanuts usually have a large head and tiny body.
Most successful show dwarfs are heterozygous for the dwarf gene. The gene makes them smaller than their false dwarf siblings, with shorter ears and blockier heads. False dwarf is the term given to a rabbit from a dwarf breed which has no dwarf gene. They are often called BUBs or BUDs (for Big Ugly Buck/Doe).
With the homozygous dominant gene being lethal, there are only three possible breeding combinations with regard to the dwarf gene: false dwarf x false dwarf, false dwarf x true dwarf, true dwarf x true dwarf.
BUB x BUD crosses will only produce more false dwarfs. They will never produce showable (true) dwarfs.
BUB/BUD x True Dwarf crosses are very common in dwarf rabbitries. Many breeders use BUB/BUDs in their breeding program when they have good type, outside their large size and longer ears. BUDs are particularly useful because they have wider hips than their true-dwarf sisters. Another reason breeders will use false dwarfs is that they have no chance of producing peanuts, because the false dwarf can never give a dwarf gene. These litters usually produce about 50% true dwarfs, and 50% false dwarfs.
True Dwarf x True Dwarf crosses can be controversial. Because true dwarf does often have smaller hips, they may have difficulty giving birth to false dwarf offspring. Also, about 1 in 4 kits will be peanuts. Another 1 in 4 will be false dwarfs, meaning that there is no advantage over the 50% true dwarf gotten from crossing a BUB/BUD with a true dwarf. The only reason this should be done is to cross two outstanding individuals.