I took a class on it, there's a reason you can dedicate a class to it, it can be a lot more complicated then you think. In one of my dairy classes we looked at a chart of expressed traits, what % is genetic vs environmental. For example, feet and legs are 70% environment and 30% genetic. There's a lot to learn, but it's interesting. One of the big take home messages is that the more traits you try to breed for, the slower your genetic process will be. The fewer traits you want to improve and focus on, the faster your progress will be.