Inbreeding

Most people have several genetic defects, but these rarely are noticed because they are recessive traits. However, it close relatives interbreed, the chance of these recessive traits matching up and causing a birth defect is much higher. Mars' population should be over 5,000 genetically diverse people to provide a healthy breeding population.

If Mars is steadily gaining more people from Earth, then this is no major concern. However, if Mars loses contact with Earth, and its population is low, then people will have to be very careful about doing everything possible to maintain genetic diversity.