Bio Ellis van Creveld
We would expect natural selection to remove genes with negative effects from a population. Individuals who carry those genes would not reproduce as much, so the genes should not be passed on. And yet we see cases where this expectation is not met. For example, human populations generally carry some disease-causing genes that affect reproduction.
University of California Museum of Paleontology
How it works[]
When carrying two copies of a gene is disadvantageous, but carrying only one copy is advantageous, natural selection will not remove the gene from the population—the advantage conferred in its heterozygous state keeps the gene around. For example, the gene that causes sickle cell anemia is deleterious if you carry two copies of it. But if you only carry one copy of it and live in a place where malaria is common, the gene is advantageous because it confers resistance to malaria.
University of California Museum of Paleontology
http://evolution.berkeley.edu/evosite/evo101/IIIE6bBadgenes.shtml
Ellis Van Crevald Syndrome[]
This syndrome is an example of how deleterious genes affect isolated populations, such as the Amish. Some of the outcomes of this include: dwarfism, polydactyly and cleft lip.
Livingstone, F.B. The Founder Effect and Deleterious Genes. (1969). AM. J. PHYS. ANTXROP.,30: 55-60.
Pinto, F. (2009). Bio Ellis van Crevald. Retrieved from http://youtu.be/cydLvNpy5ns