(genetics) The overrepresentation of a specific allele at one or more loci in a new population that arises from a small number of individuals whose small gene pool may be unrepresentative of the parental population initially or as a result of the ensuing genetic drift.
Founder effect
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(′fau̇n·dər i′fekt)
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Did You Know? In the nineteenth century, the rate of deafness on Martha's Vineyard (an island off the coast of Massachusetts) was thirty times that of the mainland population. Most deaf islanders were descendants of a small handful of English families who settled there around 1700. |
The term "founder effect" refers to the observation that when a small group of individuals breaks off from a larger population and establishes a new population, chance plays a large role in determining which alleles are represented in the new population. The particular alleles may not be representative of the larger population. As the new population grows, the allele frequencies will usually continue to reflect the original small group.
Genetic Characteristics of Founder Populations
Because the founder population is small, genetic drift can play an important role in determining the genetic makeup of subsequent generations, and allele frequencies may fluctuate. For example, consider an extreme situation where a new population is founded by just two individuals, a male and a female, perhaps because they are stranded on an island. Assume that the mother is heterozygous for a particular allele (Aa), while the father is homozygous (AA). If the couple has two children, there is a 25 percent chance that the mother will pass the A allele to both children.
If neither child inherits the allele, the a allele is effectively lost from the population. Even as they grow, many founder populations remain relatively genetically isolated, with little immigration into the population. Examples include founder populations that have remained isolated due to geographical location, such as Finland and Iceland, or due to religious customs, such as Amish and Hasidic Jewish groups.
Founder populations may have increased prevalence of certain genetic traits, including genetic disease. Disease alleles that happen to be present in the founders may be passed on to offspring, and, since the population is small, there may be a higher prevalence of the disease than in other, larger populations. Isolated founder populations, with little marriage outside of the populations, are especially likely to have a higher prevalence of recessive disorders, since parents are likely to share many genes, and there is an increased chance of inheriting two copies of a particular disease gene. Examples of rare genetic diseases that are prevalent in founder populations are Tay-Sachs disease in Ashkenazic Jewish populations and asthma in the Hutterian population.
Founder Populations Can Be Valuable for Genetic Studies
The same forces that lead to increased risk of disease also make founder populations particularly useful for identifying which genes are involved in genetic disease. Since the founder population is derived from a small number of individuals, it is likely that those individuals with a particular disease have a common genetic profile, rather than having multiple different disease mutations or susceptibility alleles. This genetic homogeneity is important, since genetic heterogeneity can make identification of any particular disease allele very difficult.
Linkage disequilibrium mapping is a powerful method for fine-mapping disease genes in founder populations. Linkage disequilibrium refers to the physical association between harmless but traceable marker alleles and a disease allele on a chromosome. The close proximity of the markers can help pinpoint the disease locus. Founder populations are particularly useful for linkage disequilibrium mapping since regions in linkage disequilibrium often span greater chromosomal distances than in general populations; that is, the disease gene will often be found with a larger set of common markers in a founder population than in a larger, more diverse population. This is expected because in founder populations, all chromosomes carrying a specific disease allele may be descended from a single ancestral chromosome, thus the disease allele will be in linkage disequilibrium with alleles at nearby markers. In a larger, more diverse population, the disease allele may have arisen on several different chromosomes, therefore the linkage disequilibrium, even for very close markers, may not be as great.
One example of linkage disequilibrium mapping in founder populations is the identification of a region containing the diastrophic dysplasia gene in eighteen families from Finland. This condition causes bent or abnormal bone growth. The region to which the disease gene was localized was narrowed substantially because scientists were able to take advantage of the extensive linkage disequilibrium around this gene in the affected individuals, all of whom shared a series of alleles surrounding the disease gene.
A related method for mapping disease genes that is well-suited for founder populations is haplotype analysis. A haplotype is defined as the set of alleles that are inherited as a group from one parent. A haplotype forms an identifiable pattern that can be used to track inheritance of all the genes within it. There are only a small number of haplotypes among the founders. Recombination tends to break up haplotypes over time, with the alleles that are closest together remaining together the longest.
A haplotype that is constantly inherited with a disease can be analyzed to narrow the region in which the gene should be sought. This means that researchers can look for shared regions or segments of chromosomes among affected individuals to help identify the location of a disease gene. For example, genetic researchers were able to demonstrate that the majority of cases of idiopathic torsion dystonia (a neurological disease) in Ashkenazic Jews were due to a single mutation from a common ancestor, because the affected individuals shared common alleles (a consistent haplotype) on either side of the mutation.
Another advantage of genetic studies in founder populations is that good clinical and genealogical recordkeeping is often available. Many genetic studies have been successful in Finland because of the population history of this region. For instance, the current Finnish population is believed to have come from a small group of individuals who settled in the southwest part of the country about 2,000 years ago. Since the initial immigration, the population has continued to be relatively isolated, with little migration into it.
Genealogical records are available through church parishes and often go back six to twelve generations, allowing scientists to develop accurate and detailed family histories linking individuals together. Despite these advantages, for common diseases such as asthma, scientists must consider that genes that cause asthma in Hutterites may or may not be relevant to other groups with asthma. Thus the scientist must weigh the advantages of performing genetic studies in small, historically isolated populations with the potential disadvantage of being unable to eventually generalize the studies' results.
Bibliography
Risch, Neil, et al. "Genetic Analysis of Idiopathic Torsion Dystonia in AshkenazicJews and Their Recent Descent from a Small Founder Population." Nature Genetics 9 (1995): 152-159.
Hastbacka, Johanna, et al. "Linkage Disequilibrium Mapping in Isolated FounderPopulations: Diastrophic Dysplasia in Finland." Nature Genetics 2, no. 3 (1992): 204-211.
Strachan, Tom, and Andrew P. Read. Human Molecular Genetics. New York: Wiley-Liss, 1996.
—Eden R. Martin and Marcy C. Speer
| Biology Q&A: What is the founder effect? |
When a small group of individuals forms the genetic basis for a
new population, only the variations within that founding group will be found in
the resultant population. This is known as the founder effect. An example of
the founder effect is found among the finches of the Galapagos Islands.
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| Veterinary Dictionary: founder effect |
Extreme genetic drift that occurs when a new population is based on only a few individuals (‘founders’). Called also founder principle.
| Wikipedia: Founder effect |
For the concept in organizations, see Founder's syndrome.
In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. It was first fully outlined by Ernst Mayr in 1952,[1] using existing theoretical work by those such as Sewall Wright.[2] As a result of the loss of genetic variation, the new population may be distinctively different, both genetically and phenotypically, from the parent population from which it is derived. In extreme cases, the founder effect is thought to lead to the speciation and subsequent evolution of new species.
In the figure shown, the original population has nearly equal numbers of blue and red individuals. The three smaller founder populations show that one or the other color may predominate (founder effect), due to random sampling of the original population. A population bottleneck may also cause a founder effect even though it is not strictly a new population.
The founder effect is a special case of genetic drift.[3] [4] In addition to founder effects, the new population is often a very small population and so shows increased sensitivity to genetic drift, an increase in inbreeding, and relatively low genetic variation. This can be observed in the limited gene pool of Easter Islanders and those native to Pitcairn Island. Another example is the legendarily high deaf population of Martha's Vineyard, which resulted in the development of Martha's Vineyard Sign Language.
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General
The founder effect is a special case of genetic drift, occurring when a small group in a population splinters off from the original population and forms a new one. The new colony may have less genetic variation than the original population, and through the random sampling of alleles during reproduction of subsequent generations, continue rapidly towards fixation. This consequence of inbreeding makes the colony more vulnerable to extinction.
When a newly formed colony is small, its founders can strongly affect the population's genetic make-up far into the future. In humans, which have a slow reproduction rate, the population will remain small for many generations, effectively amplifying the drift effect generation after generation until the population reaches a certain size. Alleles which were present but relatively rare in the original population can move to one of two extremes. The most common one is that the allele is soon lost altogether, but the other possibility is that the allele survives and within a few generations has become much more dispersed throughout the population. The new colony can experience an increase in the frequency of recessive alleles as well, and as a result, an increased number who are homozygous for certain recessive traits. A well documented example is found in the Amish migration to Pennsylvania in 1744. Two members of the new colony shared the recessive allele for Ellis-van Creveld syndrome. Members of the colony and their descendants tend to be religious isolates and remain relatively insular. As a result of many generations of inbreeding, Ellis-van Creveld syndrome is now much more prevalent among the Amish than in the general population.[5][6]
The variation in gene frequency between the original population and colony may also trigger the two groups to diverge significantly over the course of many generations. As the variance, or genetic distance, increases, the two separated populations may become distinctively different, both genetically and phenotypically, although not only genetic drift but also natural selection, gene flow and mutation will all contribute to this divergence. This potential for relatively rapid changes in the colony's gene frequency led most scientists to consider the founder effect (and by extension, genetic drift) a significant driving force in the evolution of new species. Sewall Wright was the first to attach this significance to random drift and small, newly isolated populations with his shifting balance theory of speciation.[7] Following behind Wright, Ernst Mayr created many persuasive models to show that the decline in genetic variation and small population size accompanying the founder effect were critically important for new species to develop.[8] However there is much less support for this view today since the hypothesis has been tested repeatedly through experimental research and the results have been equivocal at best.[9]
Serial founder effect
Serial founder effect have occurred when populations migrate over long distances. Such long distance migrations typically involve relatively rapid movements followed by periods of settlement. The populations in each migration carry only a subset of the genetic diversity carried from previous migrations. As a result, genetic differentiation tends to increase with geographic distance as described by the "Isolation by distance" model.[10] The migration of humans out of Africa is characterized by serial founder effects. Africa has the highest genetic diversity which is consistent with an African origin of modern humans. After the initial migration from Africa, the Indian subcontinent was the first major settling point for modern humans. Consequently, India has the second highest genetic diversity in the world. In general, the genetic diversity of the Indian continent is a subset of Africa, and the genetic diversity outside Africa is a subset of India.[11][12]
Founder effects in island ecology
Founder populations are essential to the study of island biogeography and island ecology. A natural "blank slate" is not easily found, but a classic series of studies on founder population effects were done following the catastrophic 1883 eruption of Krakatoa, which erased all life on the island remnant. Another continuing study has been following the biocolonization of Surtsey, Iceland, a new volcanic island that erupted offshore between 1963 and 1967. An earlier event, the Toba eruption in Sumatra of about 73,000 YBP, covered some parts of India with 3–6 metres (9.8–20 ft) of ash, and must have coated the Nicobar Islands and Andaman Islands, much nearer in the ash fallout cone, with life-smothering layers, restarting their biodiversity from effectively zero.
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 eighty-fold growth but minimal gene dilution from intermarriage, Quebec has what geneticists call optimal linkage disequilibrium (genetic sharing).[13] The result: far fewer genetic variations, including those that have been well studied because they are connected with inheritable diseases.
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[14] 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— which is 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, which have grown from a very few founders, have not recruited newcomers, and tend to marry within the community, exhibit founder effects. 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.[15] 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 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.[16]
Another example is the frequency of total color-blindness among the inhabitants of Pingelap, an island in Micronesia. In approximately 1775, a typhoon reduced the population of the island to only 20. Among survivors, one of them was heterozygous for achromatopsia. After few generations, the prevalence of achromatopsia is 5% of population and 30% as carriers [17][18](by comparison, in the United States, only 0.003% of the population has complete achromatopsia[19]).
More severe illnesses exist among certain Jewish groups. Ashkenazi Jews, for example, have a particularly high change of suffering from Tay-Sachs disease, a child-killer (see Medical genetics of Ashkenazi Jews).
See also
References
- ^ Provine, W.B. 2004. "Ernst Mayr: Genetics and Speciation" Genetics 167: 1041-1046.[1]
- ^ Templeton, A. R.(1979) "The theory of speciation via the founder theory". Genetics. 94:1011-38.
- ^ Hartwell, Hood, Goldberg, Reynolds, Silver, Veres, 2004, Genetics - from genes to genomes, page 688, McGraw Hill Higher Education
- ^ Raven, Evert, Eichhorn, 1999, Biology of plants, page 241, W H Freeman and Company
- ^ Cavalli-Sforza, L. L.; Menozzi, Paolo; Piazza, Alberto (1996), The history and geography of human genes, Princeton, N.J: Princeton University Press, pp. 413, ISBN 0-691-02905-9
- ^ "Genetic Drift and the Founder Effect". Evolution. Public Broadcast System. http://www.pbs.org/wgbh/evolution/library/06/3/l_063_03.html. Retrieved 2009-04-07.
- ^ Wade, Michael S.; Wolf, Jason; Brodie, Edmund D. (2000). Epistasis and the evolutionary process. Oxford [Oxfordshire]: Oxford University Press. pp. 330. ISBN 0-19-512806-0.
- ^ Mayr, Ernst, Jody Hey, Walter M. Fitch, Francisco José Ayala (2005), Systematics and the Origin of Species: on Ernst Mayr's 100th anniversary (Illustrated ed.), National Academies Press, pp. 367, ISBN 9780309095365
- ^ Howard, Daniel J.; Berlocher, Steward H., Endless Forms (Illustrated ed.), United States: Oxford University Press, pp. 470, ISBN 9780195109016
- ^ Ramachandran et al. (2005). "Support from the relationship of genetic and geographic distance in human populations for a serial founder effect originating in Africa". PNAS. http://www.pnas.org/content/102/44/15942.full.
- ^ Maji (2009). [http://www.ias.ac.in/jgenet/Vol88No1/127.pdf Phylogeographic distribution of mitochondrial DNA macrohaplogroup M in India]. http://www.ias.ac.in/jgenet/Vol88No1/127.pdf.
- ^ Thangaraj et al. http://www.biomedcentral.com/1471-2164/7/151.
- ^ genizon.com
- ^ Hey, Jody, 2005. "On the Number of New World Founders: A Population Genetic Portrait of the Peopling of the Americas" in PLoS Biol 2005 May 24;3(6):e193 webpage
- ^ McKusick, V. A.; Egeland, J. A.; Eldridge, R.; Krusen, D. E.: Dwarfism in the Amish. I. The Ellis-van Creveld syndrome. Bull. Johns Hopkins Hosp. 115: 306-336, 1964. PMID 14217223
- ^ boston.com
- ^ Hussels IE, Morton NE (1972). "Pingelap and Mokil Atolls: achromatopsia". Am. J. Hum. Genet. 24 (3): 304–9. PMID 4555088.
- ^ Sacks, Oliver (1997). The Island of the Colour-blind. Picador. ISBN 0-330-35887-1.
- ^ "The Achromatopsia Group". http://www.achromatopsia.org/. Retrieved 2007-06-13.
- Mayr, E. 1954. Change of genetic environment and evolution. In Huxley, J. (ed) Evolution as a Process, Allen and Unwin, London.
- Mayr, E. 1963. Animal Species and Evolution. Harvard University Press, Cambridge, Massachusetts.
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