That is way too complex for me to describe, you may look at some writings on the topic by a dude with pen name Greg Green.

Rusty Trichome is pretty good at genetics and OM is super smart too. They are not your only local experts.

But from what I understand a given set of parents will put out a given set of phenotypes. Through breeding programs this can be concentrated to get plants that are consistent in providing that specific phenotype.

For example, I crossed a pair, one strong sativa, just showing indica leaves, the other a stable cross and that phenotype showed mostly sativa traits with indica height. That developed 5 distinct phenotypes. I gave up at that point as I don't have room to perpetuate the whole event and stay within the law.

So, when I want a given phenotype from the seeds I have of that cross, I germinate several and only keep the phenotype of the one I prefer and toss the rest of the seedlings.