If using Speed Congenics to reduce amount of time transferring mutated lines from a one background to another (i.e. 129 background to C57BL/6 background) are samples needed from all animals produced in each generation?

• Testing is necessary only for animals that carry your gene or modification of interest. Testing 15 -20 animals from each generation will give you the best chance of producing fully congenic mice in 5 generations of backcrossing. If fewer than 15 samples are sent per generation, the number of backcross generations required to obtain a fully congenic line will be increased.
• Please note that sex chromosomes must be fixed before testing starts. The best method to fix the Y chromosome is to cross female carriers to wildtype males of the recipient strain to produce F1. The X chromosome is then fixed by mating F1 carrier males to wildtype females of the recipient strain. The offspring of this cross and all subsequent back crosses should then be tested to select the most suitable males for the next back cross.