In general, coding regions are more likely to have the same sequence. It is more likely to produce a non-functional gene product (protein).
Non-coding mutations are free to mutate because it has no protein product, and are less likely to affect phenotype. They are neutral mutations. Different mutations can be differentiated even when different groups have the same coding sequence.
MRNA - messenger RNA - complementary to DNA, it is formed in the nucleus. Unlike DNA however, it can move outside the nucleus.
tRNA - transfer RNA - a unique structure, at one section there is an anticodon, a series of 3 bases which are complementary to the code embedded on the newly formed mRNA. Additionally, another section has an amino acid attached, this is essential for protein synthesis.
rRNA - ribosomal RNA - forms ribosomes. Ribosomes are essential for protein synthesis. The newly formed mRNA leaves the nucleus and attaches to a ribosome. Upon attachment, the ribosome mediates the formation of a primary structure protein. The tRNA binds to the mRNA forming a codon-anticodon complex, this allows bonds to form between adjoining amino acids (attached to tRNA), this complex moves through the ribosome, which detaches the newly formed amino acid strand - thus forming a primary structure protein.
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.