I've been playing around and inbreeding the same line for the last 3 years. I'm at f3 stage now, I wouldn't have gotten here if there weren't genetics worth keeping.
In the f1 polyhybrid I found that "keeper" plants were plentiful, I got about 15 that were worth keeping from that batch of 45(about 24 females), with 6-7 being excellent, and 1 of those being some of the best I've ever smoked. It's the reason I inbred the line further, if you can manage to find keepers in a line, just select you favorite phenotypes and and keep select further down you will end up producing a more homogeneous chemotype/phenotype. The more parallel familial lines you can maintain the better for the genome(preservation-wise)
I selected from my f2 generation's best sativa expressions and produced a bunch of hybrids and f3's I am testing now, you only know if a male has the goods by growing it's offspring.
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Very soon I will be testing this "Peace Train" hybrid male to see if he passes on his 'Purple' Haze traits to offspring.
That's how I decide if a male is a "keeper", does he give you the goods?
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I try to go through as many seeds as possible.