Final answer:
The descendants on Venus will display the founder effect, which leads to a higher prevalence of certain genetic traits within a population, as seen in the higher frequency of polydactyly among the Amish due to their founders' genetics.
Step-by-step explanation:
The descendants of a man and a woman originally from southern Ukraine who are the only survivors of a colony on Venus will show the effect of founder effect. This genetic principle refers to the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. An example of this is the higher frequency of polydactyly among the Amish population, which can be traced back to a ship's captain with this trait. The founder effect can lead to increased prevalence of certain genetic traits within a population and can be a significant force in the evolution of the population.