What Is Inbreeding?
Inbreeding is when a certain horse, for example, Rain Dancer, shows up in a pedigree two or more times.
If you count the foal as 0, and then work backwards through generations, this example would be 2 x 3 to Rain Dancer. Anything less than that is gross and probably deformed, so the SIM doesn't allow anything closer than 2 x 3 or 3 x 2, which is an example you can find in some modern pedigrees (Quiet American, sire of Real Quiet, was 3 x 2 to Dr. *).
What Are the Pros of Inbreeding?
Inbreeding gives a double dose (or more) of a powerful horse. Some people love inbreeding to the same mare, such as 3 x 3 to Pike Place Dancer, an excellent blue hen. Or sometimes it's to a stallion, such as 3 x 3 to Fighting With Wit, to give strong stamina on both sides.
When you want to get creative, you can inbreed to full or half sisters/brothers, such as a pedigree like this:
In this case, your horse is 3 x 1 to full sisters Cascade and Champagne, who are both Priceless Forever x Rain Dancer. (This means they are 4 x 2 to Rain Dancer and Priceless Forever, and this particular example has another Rain Dancer line as well).
What Are the Cons of Inbreeding?
It doesn't always work and can sometimes turn out so that the horse inherits only the worst genes from that parent - for example, inbreeding to stamina might end up getting a plodder, or inbreeding to full brothers who are questionable sires can get a dud in the shed.