Explanation- Females of breed A are crossed to males of breed B. Their female offspring (F1) are (back) crossed with a male of breed B. Their female offspring (F2) are crossed with a male of breed A. Their female offspring (F3) are crossed with a male of breed B etc. Alternately in each generation males of breed A and breed B are used and crossbred females in all generations can be used to produce replacements. 
