Honestly, Eugenics movements represent a fundamental misunderstanding of how genes work. They're a recipe, not a blueprint. See, what happens in cells is that strips of DNA are translated to strips of RNA which then travel outside the nucleus. The structure of DNA is familiar. The rungs of that ladder are "bases". Each group of three bases codes for a particular amino acid or sends a command such as start and stop. These amino acids are what make up proteins.
Precisely which strips of DNA are used in the first place is determined by the chemicals in the cell environment and outside it. Also, there are a lot of pretty damn cool chemical machines within the cell which can twist improperly coded proteins so that they can still do their job despite flaws in the DNA leading to flaws in the protein itself. Cells grouped together affect each other, the shapes that they form as tissues and the roles they play.
It's not really possible to take a cell, extract the DNA and say "this is the organism it will produce!" because it's so environmentally variable. There's no guarantee that the same DNA run twice will produce the same result. Likewise it's impossible to isolate genes for personality and the like as so much of this is a result of socialisation. Values are much the same.
However, hereditary susceptibility to diseases like cancer can be found, but that's a specific case where the disease results from flaws within the DNA which mean that cell replication is unlimited. This can be passed on because it's bound up as a part of chromosomes. Trisomy-21, otherwise known as Down's Syndrome is a genetic disorder when a third copy of the 21st chromosome is present. Passing on of Down's Syndrome is more complicated due to the manner in which chromosomes divide to produce sperm and egg. For other things like mental illness it's much harder because of the recipe-role of DNA mentioned above.
So when Eugenics comes along and claims to breed out diseases it mistakes the mechanics of what it's doing, how genetic susceptibility works. Selective breeding is a relatively clumsy process, for all it can produce very dramatic results. It does depend on what one is looking for. I'll use hair colour as the example here. There are two 'spots' for genes which code for hair colour. Both of these can be dominant (two copies of the dark hair gene, dark hair in individual), both can be recessive (two copies of red hair gene, red hair in individual), one could be dominant and one recessive (one for dark, one for red, dark hair would show) or there could be a situation of co-dominance in which the characteristics of both genes show (roan cattle where both types of hair exist). If one is trying to breed for say, red hair, which only shows up when there is no copy of a dominant gene for hair colour present, then it can manifest after generations of dark-haired people because no-one knows that they have the recessive gene. One can take only creatures with red hair to breed which would guarantee the eradication of the dark haired gene as it would not be able to carry on to children, or one can produce red haired creatures from dark haired creatures which have the recessive gene as 1 in 4 of the offspring of two of these would only possess the recessive genes. Eradicating recessive genes is much more difficult because unless you have a "pure-breeding" strain of many generations, there's no guarantee that both copies of the gene for say, hair colour, are of the dominant one.
Not to mention that mutations can always make things crop up again.
