I wouldn't consider it a backcross if its crossed to other strains. To be a bx it would have to be backcrossed to itself.
any filial interbreeding is an Fgeneration...f1,f2..
When a filial progeny of the parents is crossed back to one of the original parents it's a backcross.
you cross an f-5 with an f-3 that's backcrossing, even though you don't use the original mom you went "back" in your selection process. The plants must be of the same filial generation to progress to next.
Bx2 and Bx3 would be putting the each future generation of offspring back to the original mother plant. F2 F3 F4 would be breeding each future generation with itself ie. putting an f2 to another f2 would make an f3 and so on. Now putting say an F5 to an F2 would still be make an f3 if im not mistaken....but it may be considered a bx....thats where the grey area comes in.
any time you go back its a back cross. if you don't go to the original parent it would be an IX i guess or still just a back cross???. so if you take a f1 male and hit the original female mom it would be a bx1(back cross 1st gen) if you take these bx1 kids and go back to the original mom again makes the bx2, but if you take say these bx2 kids and go back to say the bx1 instead of the original mom im thinking IX(in cross) even though you went back.
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