Ah now you made me feel self conscious about opening Pandora's box.
So truly there are things you don't want to breed unless you have excess space and don't care need or want to use the plant. Just slapping two plants together could be real bad or real great.
I learned about plant genetics a long time ago from this horticulturalist but I just googled the history of this and found this Monk who discovered how genetics works but died long before his papers were read and ever appreciated and he did breeding experiments that clearly showed why you must be careful what you breed.
Many people including Darwin were wrong about genetic traits and how they were passed down but this guy figured it out. So I will explain how I think about it and leave you with a link I found that explains the very most basic aspects. There are long thick college course books on this that are very expensive. The knowledge is on the internet but it would take me days to splain it all.
So here goes the short version.
Lets start with a chart I stole from the link below.
So what we see here lets say is a mapping of how genes work in breeding both animals and plants. Lets say this gene controls the color of the seed. Seed could be Striped which will be GG or just a tanish brown which is YY.
Now if you breed them together he found that you get as a result for the First Generation that a true cross between them is formed.
But subsequently. If you breed the F1's together you can get anything from very striped to very yellow. And it only gets worse because the gene will pass on the ability to pass on traits but not show them.
So the point is.
To breed for traits you cross two likely candidates that have strong genetic backgrounds that are not variable that if you picked up any two seeds from the bag and breed them they would produce an identical offspring. Those are called Land race strains and are so named because they grew for so long in one place they have a consistent genetic profile. Northern Lights is a well known land race that is so old that you get the same thing every time. Then you take that and cross that with another well known land race and create an F1 that is a true cross of the 2. F1's are know for vigor and grow faster and are very consistent. So all of the seeds from that breeding will be very similar. They
may take on all the best traits of the parents or
all the worst. So crossing 2 plants may not result in a great plant. Or it my result in a plant that grows hearty fast vigorous with big bulky hefty flowers that are low in potency.
So once you found an F1 you like that has some trait you are going after you have to stabilize this. You need to back breed this trait into the parent line of one or both of the parents to make this a dominate trait. It takes many generations. The fastest path is 6 generations I think. 10 is a better path. But it is very specific. You can't just cross breed randomly and get a stable strain. If you just keep breeding without going back you will just create random noise that may have some real cool once off never repeatable stuff here and there and the potential for some real nasty stuff too.
So for example....
I have in there Raspberry Cough (F1), White Widow(Stable Hybrid), New York Power Diesel(F1), (I had 2 more strains I started with but were males or failed to sprout).
RC is an F1 of a Cambodean Landrace x ICE (Stable classic Indica mix including Afghan and NL)
NYPD is an F1 of Mexican Sativa x Aurora Indica (NL x Afghan)
WW is a stable hybrid that is a cross of Brazilian Sativas and South East Indian Indicas.
So If I mix the WW Polen with the NYPD or the RC I could be getting anything under the sun as they have nothing in common. Now If I had some NL pollen I could back breed the RC or the NYPD to the NL and have a new version of NL that is cool and after a few more generations of selective breeding you could have something stable.
If I had a Land race Afghan I probably could back breed both the NYPD and the RC and do some selective breeding there and take those results and cross them for something wild.
But it gets real complicated. So edumacates yerself firs
Basic Principles of Genetics: Mendel's Genetics