|MadSci Network: Genetics|
Though I can't claim to be an expert in this field, it may be worth doing some searches on the web for Dyson Spheres. One form of Dyson Sphere is a hollowed out asteroid which is used to transport people from A to B where the distance is so great that none of the people who left, or any of the next few generations, would not actually be alive when it reached it's destination. These typically had tens of thousands of people on them.
Try the links in the MadSci Library for more information on Genetics.