Backcross
From Wiki Weed
Backcross (BX)
A backcross is a hybrid plant that has been bred with one of its parents (or a plant that is genetically similar) in order to create offspring that is closer to that of the the original parent. For example, a grower could breed a plant with its own father to make sure the baby has its dad's height. This is often done to maintain rarer strains or strengthen those with desired recessive genes.
