Thanks - looks like I was wrong about the etymology. I'd read the term itself was created in response to what the nazis did, a lot of which started under the guise of eugenics.
This book spent some times talking about it and I thought it was interesting: http://www.simonandschuster.com/books/The-Gene/Siddhartha-Mu...
You've got the etymology confused. 'geno-' comes from Grk 'genos', which means race/tribe. Genocide refers to ethnic killings. Eugenics ('eu' good + 'gen-' race/tribe) refers to the goal of achieving 'better races' and methods to achieve them. Unfortunately those methods involve pruning perceived bad seeds.
They come from the same root word, but genocide isn't derived from eugenics.