Periodically, a few individuals decide to strike out on their own and set up a community away from kith and kin. The founders of this new community — be they rats, bats, bears, or humans — determine the genetic makeup of the new population. Founder effects refers to their influence in this regard. Understanding founder effects helps scientists understand how the genetic makeup of original members of the new community affects the gene frequencies in subsequent generations.
Every time a few individuals separate themselves from their original population and move to a novel environment, you can see how founder effects can be important in determining allele frequencies. Quite a bit of variation in human blood type occurs across different geographical regions, for example. Some areas have a preponderance of one blood type; other areas have a preponderance of another type. Founder effects, spurred by human migration patterns that often involve small groups founding new populations, can explain these differences, as the following sections explain.
Ten thousand years ago, a small group of humans crossed the Bering Strait and colonized North America (and subsequently South America). Genetic drift probably is the cause of the very low frequency of the type B blood allele in Native Americans (4 percent), even though it is much more frequent in people from Asia — the presumed source of the Native American colonists.
The Amish population was founded by a small group of Europeans in the early 16th century. In the 18th century, the first Amish migrated to the Pennsylvania colony in the New World.
The Amish, who remain almost entirely reproductively isolated from the rest of America, have a distribution of blood types different from those in the rest of the United States or in Europe. Additionally one of the original couples carried the allele for polydactily (extra fingers or toes) which has increased in frequency in the small Amish population by chance; they just happen to have more than the average number of descendants.
Okay, this example isn't for real. But if you've seen the 1950s-era sci-fi flick When Worlds Collide, you can imagine founder effects in action. In the film, Earth is doomed by the approach of a runaway planet. Humanity is saved when a group of scientists and engineers (including the requisite cynic-turned-true-believing hero, the brilliant-and-self-sacrificing old scientist, his beautiful-but-prone-to-melodramatic-emotional-displays daughter, and the steady-but-dull-as-dishwater-and-soon-to-be-spurned boyfriend) gets together to build a spaceship large enough to carry a few humans, a few head of livestock, a few plants, and one adorable dog to another planet to begin life again. Think of the genes this bunch has to work with! Their blood type — given that they left the riffraff on Earth to perish — definitely would be blue.
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