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Founder effects in human populations
Due to various migrations throughout human history, founder effects are somewhat common
among humans in different times and places. The effective founder population of Quebec was
only 2,600. After twelve to sixteen generations, with an eightyfold growth but only minimal gene
dilution from intermarriage, Quebec has what geneticists call optimal linkage disequilibrium
(genetic sharing).[15] The result: far fewer genetic variations, including those that have been wellstudied because they are connected with inheritable diseases.[clarification needed]
Founder effects can also occur naturally as competing genetic lines die out. This means that an
effective founder population consists only of those whose genetic print is identifiable in
subsequent populations. Because in sexual reproduction, genetic recombination ensures that with
each generation, only half the genetic material of a parent is represented in the offspring, some
genetic lines may die out entirely, even though there are numerous progeny. A recent study[16]
concluded that of the people migrating across the Bering land bridge at the close of the ice age,
only 70 left their genetic print in modern descendants, a minute effective founder population—
this can be easily misread as though implying that only 70 people crossed to North America. The
misinterpretations of "Mitochondrial Eve" are a case in point: it may be hard to explain that a
"mitochondrial Eve" was not the only woman of her time.
In humans, founder effects can arise from cultural isolation, and inevitably, endogamy. For
example, the Amish populations in the United States exhibit founder effects. This is because they
have grown from a very few founders, have not recruited newcomers, and tend to marry within
the community. Though still rare, phenomena such as polydactyly (extra fingers and toes, a
symptom of Ellis-van Creveld syndrome) are more common in Amish communities than in the
American population at large.[17] Similarly, there is a high frequency of fumarase deficiency
among the 10,000 members of the Fundamentalist Church of Jesus Christ of Latter Day Saints, a
community which practices both endogamy and polygyny, where it is estimated 75 to 80 percent
of the community are blood relatives of just two men - founders John Y. Barlow and Joseph
Smith Jessop.[18]
In 1814, 15 British colonists founded a settlement on Tristan da Cunha, a group of small islands
in the Atlantic Ocean, midway between Africa and South America. One of the early colonists
apparently carried a recessive allele for retinitis pigmentosa, a progressive form of blindness that
afflicts homozygous individuals. Of the founding colonists' 240 descendants on the island in the
late 1960s, 4 had retinitis pigmentosa. The frequency of the allele that causes this disease is ten
times higher on Tristan da Cunha than in the populations from which the founders came.
Cleidocranial dysostosis is found among the South African Muslim population due to a Chinese
man who was affected by this disease.[19]
More severe illnesses exist among certain Jewish groups. Ashkenazi Jews, for example, have a
particularly high chance of suffering from Tay-Sachs disease, a fatal condition in young children
(see Medical genetics of Ashkenazi Jews).