Here is a quote on an article about breeding horses for instance:
“Perhaps the greatest advantage of inbreeding is that it increases the prepotency of individuals within a herd and consequently helps to create distinct true-breeding strains or families. This prepotency (the ability of a stallion or broodmare to stamp desirable characteristics upon their offspring with a high degree of predictability) is the result of the parent being homozygous for important desirable traits. When such a parent carries two identical alleles on corresponding points of a chromosome pair, he transmits that allele to the same chromosome point within his offspring. If two such parents are mated, the offspring will always possess the same desirable trait. Therefore, as inbreeding increases homozygosity, it also enhances prepotency.
This applies to humans as much as it does to any other living thing on this planet.
If all your great grandparents were smart, your grand parents were smart, your parents were smart, YOU and all your siblings will be smart. That is simply the rules of breeding.
Now if one of your grandfathers was an idiot, but married a smart wife, around half of their children might be dumb and the other half smart. If the smart children married other smart children most of their children would be smart, BUT they could still produce some dumb children because of the grandfather was an idiot remember?
This is what confuses people.
It takes MANY generations to breed out an undesired trait, and at each generation you have to be sure your not undoing the previous progress by breeding with someone who carries the undesired trait.
Sounds like a process to create GMOs. ;-)
Never forget: the only way to breed OUT an undesirable trait, is to remove the individuals with that trait from the breeding pool.