Oh, it is a complicated subject; but it is one that you can get your brain around. Let’s start with two terms – landrace and hybrid. These two categories are how cannabis comes; landrace strains are the relatively few that have evolved, pretty much on their own, in their native lands and hybrid strains are those that humans have been breeding. The most important types of cannabis are indica and sativa – the hybrids are combinations of these.
When you start breeding, you want your parent specimens (P1 and P2) to be as pure as possible – and this is really not always possible. So, it is very important that you breed for two or more characteristics or traits – potency, odor or color, etc.; but don’t try more than that on your first round. The seeds from your first round will be F1 and they will show all the dominate traits of the two parents. But, F1 is unstable and tend to not breed true as F2. You must pick out the traits you are isolating from the F1 line – say, a purple male and a purple female; by crossing plants with similar traits, you increase your chances of having a purple offspring.
Now you have an F2 and you are starting down the path to a stable strain. As you continue to cross for similar characteristics as your create new generations of cannabis, the F3, F4, F5 and more will become more and more reliable – eventually, your crossing will always show the purple characteristic.
A tip; clone your original P1 and your F1, F2 and so on. If there is a characteristic in the F1 that you do not want to lose, cloning will help you keep it. This is not breeding – which always involves pollen. You can also back cross your F1 or F2 into your P1 clone.
Do not rest on your laurels. Watch very carefully that your characteristic is expressing. And, be patient as it takes several seasons to achieve your breeding goals.
This has been real down and dirty. Dig around on the web for more information. Google “ cannabis breeding” and check out the images that come up. All those little breeding diagrams ought to make some more sense now.