Inbreeding depression
During settlement planning, if the human population is to reproduce, careful consideration into the size of the starting population must be carried out. The size of the human population must not be too small due to the risk of Inbreeding Depression (causing genetic disorders) and it must not be too big due to the risk of overpopulation (causing lack of food/resources).
To achieve a big genetical diversity an idea is to bring a few adult colonists and many fertilized human eggs to Mars. It gives a chance to build a big community on Mars with a comparetively small transport effort.