Alright, i'll try to be quick with this. Everyone has two chromosomes of each type. They aren't the same though, each chromosome holds different data. When you breed, one of your chromosomes matches up with one of your mate's chromosomes. There's a 50% chance you will give your #1 chromosome, and 50% the other. So we'll call this kid, kidA, and it has #1. Then you breed with kidA. You have a 50% chance of giving chromosome #1 to the new kidB, and so does kidA have a 50% chance. So kidB has a 25% chance of getting two #1's for this chromosome. KidB also has a 25% chance of getting EVERY individual chromosome he has to be the same. To get more than one at a time, the probability builds up, but it's still likely. THIS is bad! This is the reason why imbreeding is bad, and what causes problems. Having two chromosomes that are the same is a very bad thing. There's tons of different reasons why this causes problems, but that's the basic idea. Though if kidB breeds with new blood, kidC's blood will be fine.
Hope that helps, if you want to know more of the problems that imbreeding causes, just look them up in a gentic book.
Hope that helps, if you want to know more of the problems that imbreeding causes, just look them up in a gentic book.