No you can backcross to a sibling of one of the parents, I would call it an IBx poly hybrid.
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NO, you cant call it backcross. Because its not a backcross. Backcrossing is the idea of isolating genetics while incrossing is the idea of finding traits you like from both siblings.
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Yes, cannabis doesn’t have genes like every other species on Earth, so it follows different rules and magically gets inbred faster… or maybe it’s just that cannabis breeders don’t care about doing it right and want it done fast instead. 
Yes, it’s faster… F/S3 though? I don’t think so, unless you can actually test what alleles are being passed down with a more sophisticated method than just growing the plant and observing it with our basic human senses. You seem to be assuming that 50% of the chromosomes will be homozygous for each individual breeding, and more to the point, that you’ll know which are the homozygous ones. With perfect selection though, you’re right, it will only take 2 generations of inbreeding. With perfect selection, it only takes 3 generations of inbreeding to achieve an isogenic population even starting with fully heterozygous parents. Know anyone around here who can do it perfectly? I don’t…
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Yeah, you got some point in what you are saying. As I said i wouldnt say that you reach 100% doubling at S3 but done right you should be close to 100%. Even at 75% you will induce vigor in the offspring cultivar. For every pair you can double, If the other strain also got a Double at that pair it will induce some vigor.
I dont either believe in a perfect world but this type of breeding works Great with what we home breeders have atm.
Edit: this only works with selfing, so only S generations and not filial generations.
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S1/2, etc. are nomenclature only used in cannabis, so since we were talking about breeding in general I opened it up a little… but technicalities aside, it sounds like we’re mostly agreeing. 
The third generation will definitely be enough to stabilize it some, and if you’re doing it for yourself, I certainly don’t begrudge you doing it slightly less than perfectly. Since you specifically made the point about true F1s only coming from IBLs, though, I figured clarity was best… and since this is about giving advice to others, not the choices we’re making ourselves. I certainly won’t be breeding any of my strains to F150 to make true IBLs. 
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I used to breed lovebirds, and for them you can you can homogenize them in not to many generations, like two green ones got a small chance of producing a yellow or even an albino(rare) and if you keep coupling yellow ones together in like “f7”, 100% of the offspring are indeed all yellow and your not even inbreeding, because in bird it causes deformities and infertility.
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Yeah, i do agree with you and I would love If more breeders start to sell S3 beans for you to cross to other stuff. To make you own F1s😊
F1s are the offspring of parents with large Amount of chromosome doubling. Even If you got a 100% Double chromosomes. It wont be a 100% genetical identical between siblings because of evolutionary mutation.
Pz
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Wow i love birds, got a couple of cockatiels myself. Havnt breed with them yet 
Lets say (i dont know IF its like this in brids) one chromosome is responsable for fether colours. You only need to Double that chromosome to achieve 100% of the same colour in the offspring.
Pz
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Yup, it doesn’t take 7 generations more like 5. Love cockatiels too. I traded them all and got a amazonian parrot he is so funny he must be 5 months old. Now the breeding couple produced green(common), yellow and even blue babies. Whoever got them is lucky they were really good.
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That’s exactly the same process we’re discussing here. 
As @LonelyOC said, assuming the feather color is based on one chromosome and not linked genes, and discounting mutations, getting the offspring to the point where they’re 100% yellow is getting one chromosome to be homozygous. Inbreeding them to the point where they’d be considered an IBL, as you say, can cause deformities and infertility as the negative recessives start to stack up on homozygous chromosomes. There’s a reason why some dog breeds have heredity heart conditions, or back problems, or any number of other medical issues… it’s because we inadvertently bred them for it.
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Well purple caper does stuff like that and calls it a bx, and they got Ed rosenthal backing them up.
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IX just means they didn’t out Coss to a totally different strain. But an example would be they could have bred an F5 with an F1. It’s a very vague term.
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In animals, what you call backcrossing is backcrossing. But in plants its to cross something back to a parent. Hope it clears your confusion bro
Maybe they are breeding animals 
Pz
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IX Can be done between siblings and grand siblings. As you said, its a cross within the parental line.
Pz
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They need to come up with a new term for that, I would just call it a PBx = parental back cross.
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Well idk. In university we always called it backcross and If some of the animal students talked about backcrossing you understood that it was not breeding to the parent.
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I like you lonely. You Seem well educated, compared to most people I talk to.
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Wow thank you 
Thats very kind of you to say. I got into university in botany here in Sweden because of weed. I had fantasys that weed would become legal here and I would make my own Swedish weed brand. But i started to code instead and now i work as a software developer. I know i dont know everything, but the stuff i do know i like to share to others and of they share what they know, i learn even more. Its fun to learn and its very rewarding to help others. Spread the love and Spread the wisdom of cannabis and nature.
Pz
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I feel you I’m in a EU country where it’s illegal, i dream about opening a business when things change. But i don’t think it will and if does they will put a cap on thc values.
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Yeah, i know it will become legal in the Future. But i want to move to Canada or Barcelona, but all the raids in Barcelona atm makes me scared. So me and the wifey will probably move to Canada. She needs to smoke for her health problem and being scared all the time takes a toll!
Tbh i wouldnt care If they put a thc cap or plant count. Atm i just want to be able to grow, breed abit and smoke without being scared
Pz
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