two separate plants (P1), are crossed, the resulting generation is an F1, if those are crossed, they will create F2, etc., etc., ... F1 is fairly stable, F2s can show a wide variation of different phenotypes ... thorough back-crossing to the original parents thru several generations will 'settle down' the differences, and eventually stabilize the strain, allowing further generations to pretty much give identical offspring ... some strains never fully stabilize ... the differing phenos are not necessarily inferior, just different from one another ... it's harder to grow plants of varying heights and characteristics together indoors, that's one problem that presents itself ... :smokin:

P.S. ... good luck with your Willie ... go light on the nitrogen, she's easy to over-feed, very tall, top her early, if you don't have a lot of height, (my cabinet stand 7.5-feet tall) ... I flowered mine for 14 weeks, and still never got a single amber trichome :wtf: ... my outdoor Willies are MUCH better, I really wouldn't recommend it for indoors, if you have a choice ... (Reeferman Willie Nelson = never again) ... :wtf: