Article: Want to Colonize an Alien Planet? Send 40,000 People
One of the major obstacles to colonizing a distant planet has to do with genetics.
It would take at least 150 years to get from Earth to some other, colonizable world and thus at least 6 generations.
In order for the colony to be viable in the long term there has to be enough genetic variation to avoid what is called the "founder effect" or a "genetic bottleneck." Only having a small population to start with concentrates "deleterious" genes due to in-breeding.
The authors estimated that about 40,000 people would be necessary for such a long-duration space flight and for colonizing the world.
It would help if the population chosen would be made of people of diverse ethnic/racial backgrounds.
One way in increase genetic diversity, not discussed, would be send along a large number of any of the following: sperm samples, frozen eggs, or embryos.