George, mostly E is a condition- a result of trauma or infection.- not an 'illness' caused by genetic mutation and passed down.
Having said that, children of mothers with E have a 3-9% and of fathers 1.5-3% chance of developing it after such events, and the usual chance is about 1%.
Recently, a few types of seizure conditions have been linked to genes, and it is thought these genes maybe causing neurons to be more prone to excitability and thus more prone to seizures.
The 'problem' with genes is that while having 1 set of genes may reduce survivability, some of the genes in that pattern, when grouped with other genes, may cause something beneficial to it!
There is a concept in evolution known as trade-offs. If having 1 set of genes causes a benefit which leads to being more prone to survive, those genes will stay- even if 1 or more of those genes when combined with other genes present, does not. As long as the probability of survival is slightly greater than the probability of not surviving, they'll stick around.
Just because a person is more prone to E, it does not mean that they won't be able to reproduce and thus potentially pass on the genes that make them more prone to it, it may just reduce the probability slightly.
One might argue that the very gene mutation that causes excitability of neurons which may lead to greater susceptibility, may also provide (lets say faster fight/flight responses, or better capability of lateral thinking for example) and thus be beneficial.
I hope that makes sense. IANAD so don't shoot me if that info isn't 100% accurate
