When creating your own strain from seed, you always want to make sure you choose the mother and father plants that have the dominant recessive traits you are looking for. What this means is, just like in people, plants pass along certain traits as well. If a father has red hair and real pale skin and the mother is blonde with pale skin, chances are the kid is going to be blonde or redhead with pale skin. If pale skin is what you are going for in a child then I suppose you would try to find a partner that has pale skin as well.
This is the same in plants.
Male marijuana plants usually tend to pass along high THC or CBD potential to their seeds when females pass along plant structure, bud design or density and other such variables. When creating a strain, you would choose a female plant with the structure you want. You would plant multiple seeds until you had a good sense for the chances of variable.
Chances of variable are what I call the chance that this certain trait will happen again to an un-cracked seed in my hand. First seed: short and stout but second seed: tall and lanky... What are the chances the rest will be short and stout? Once you figure out how repetitive a trait is, you can now place it numerally by dominant recessive-ness. Eight plants had big fat leaves and were short and stout and only two were lanky and had narrow leaves. One can assume the chances are the next two seeds will be short and stout.
If you chose a female from a group of seeds that came from a plant with a lot of variable traits, then chances of picking the one with the most dominant-recessive traits is slim to none. This is why it is good to choose genetics carefully and do a cross or two before you select the one female you want. Get a feel for the genetics and what passes to the new seeds. I always start with regular seeds in my garden and select the male and female out of 50-100 seeds. I choose two or three males and females and I group test them.
I clone the males to take note of a hollow stem. Hollow stems are signs of high THC due to the easy ability of the hollow stem to transfer THC through the plant. Now, see how fast the clones root. Once all rooted, put in dirt for a week and then switch to 12/12. How fast do they grow pollen? I typically choose males that are fast producers to pass this trait to the female to keep harvest times down. Which plant yielded the most pollen? A little bit goes a long way, so be careful. Pollen is easily sterilized by water.
Always keep a water bottle on mist near male plants to avoid pollinating whole crops in different rooms or even miles away. I mist my body and the air around the male chamber and the hallways and everything I touched. This makes sure I don’t pollinate my females or accidentally pollinate with a plant I am not choosing to use!
Now I repeat this with the females. I cut clones and choose the two females that bloom quick, root quick, grow healthy and strong from day one. Females pass along structure and strength along with yield! Choose the small clone that yields the most!