Question

Ellis-van Creveld syndrome is a form of dwarfism that is inherited in an autosomal recessive pattern. This particular form of dwarfism occurs quite frequently in the population of Amish living around Lancaster County, PA and can be traced back to one couple, Samuel King and his wife, who came to the area back in the 1700s. In this specific Amish population, the allele frequency is 7%, compared with 0.1% in the general population.
Part A: What is genetic drift? (1 point)

Part B: Identify the effect that is responsible for the genetic drift that has occurred in this population? (1 point)

Part C: How does isolation of a population lead to a decrease in genetic diversity? (2 points)

Answer 1

A) A textbook definition would be that genetic drift is: a random change in allele frequency caused by a series of chance occurences that cause an allele to become more or less common in a population. In layman's terms, this means that genetic drift happens when luck makes the genetic pool of the population to deviate from what is expected.
B) The cause for this genetic drift is the aformentioned couple. Because amish communities are small and they select partners from their community, having even a couple of carriers of alleles in a community can make the allele freuency much larger than expected; for example, if the community was 100 persons, the percentage would be in the order of 1%, still much larger than the general population. Thus, the cause here is that a small population had a couple of carriers.
C) Sexual reproduction leads to a mixing of alleles from both mother and father and helps diversity. When a population is isolated, the gene pool is fixed and no new genes can come in, reducing diversity. Also some people that have an allele might die, hitting diversity even more. Finally, having a small population creates a strong pressure in some circumstances that leads to elimination of some traits and diversity.