Infinite alleles model

Infinite alleles model

The Japanese geneticist Motoo Kimura and American geneticist James F. Crow (1964) introduced the "infinite alleles model", an attempt to determine for a finite diploid population what proportion of loci would be homozygous. This was, in part, motivated by assertions by other geneticists that more than 50 percent of "Drosophila" loci were heterozygous, a claim they initially doubted. In order to answer this question they assumed first, that there were a large enough number of alleles so that any mutation would lead to a different allele (that is the probability of back mutation to the original allele would be low enough to be negligible); and second, that the mutations would result in a number of different outcomes from neutral to deleterious.

They determined that in the neutral case, the probability that an individual would be homozygous, "F", was:

:F = {1 over 4 N_e u + 1}

where "u" is the mutation rate, and "N"e is the effective population size. From this it is possible to determine an upper limit to the number of possible alleles in a population, "n" as the inverse of the homozygosity:

:n = {1 over F} = 4N_e u + 1

If the effective population is large, then a large number of alleles can be maintained. However, this result only holds for the "neutral" case, and is not necessarily true for the case when some alleles are more or less fit than others, for example when the fittest genotype is a heterozygote (a situation often referred to as overdominance or heterosis).

In the case of overdominance, because Mendel's second law (the law of segregation) necessarily results in the production of homozygotes (which are by definition in this case, less fit), this means that population will always harbor a number of less fit individuals, which leads to a decrease in the average fitness of the population. This is sometimes referred to as "genetic load", in this case it is a special kind of load known as "segregational load". Crow and Kimura showed that at equilibrium conditions, for a given strength of selection ("s"), that there would be an upper limit to the number of fitter alleles (polymorphisms) that a population could harbor for a particular locus. Beyond this number of alleles, the selective advantage of presence of those alleles in heterozygous genotypes would be cancelled out by continual generation of less fit homozygous genotypes.

These results became important in the formation of the neutral theory, because neutral (or nearly neutral) alleles create no such segregational load, and allow for the accumulation of a great deal of polymorphism. When Richard Lewontin and J. Hubby published their groundbreaking results in 1966 which showed high levels of genetic variation in Drosophila via protein electrophoresis, the theoretical results from the infinite alleles model were used by Kimura and others to support the idea that this variation would have to be neutral (or result in excess segregational load).

References

*cite journal|author=Kimura, M. and Crow, J | year=1964 |title=The Number of Alleles that Can Be Maintained in a Finite Population | journal=Genetics |volume=49| pages=725–738
*cite journal|author=Lewontin, R.C. and Hubby, J.L. |year=1966 |title=A Molecular approach to the study of genic heterozygosity in natural populations. II. Amount of variation and degree of heterozygosity in natural populations of "Drosophila pseudoobscura" | journal=Genetics |volume=54| pages=595–609 [ [http://www.genetics.org/cgi/reprint/54/2/595 A MOLECULAR APPROACH TO THE STUDY OF GENIC HETEROZYGOSITY IN NATURAL POPULATIONS. II. AMOUNT OF VARIATION AND DEGREE OF HETEROZYGOSITY IN NATURAL POPULATIONS OF DROSOPHILA PSE... ] ]


Wikimedia Foundation. 2010.

Игры ⚽ Поможем сделать НИР

Look at other dictionaries:

  • Infinite-alleles model — Unreferenced|date=May 2008The infinite alleles model is used to examine the dynamics of mutations. It is built on a set of assumptions:* Each mutation is unique * Mutations are not reversibleThis means that two alleles identical by state must… …   Wikipedia

  • History of molecular evolution — The history of molecular evolution starts in the early 20th century with comparative biochemistry , but the field of molecular evolution came into its own in the 1960s and 1970s, following the rise of molecular biology. The advent of protein… …   Wikipedia

  • evolution — evolutional, adj. evolutionally, adv. /ev euh looh sheuhn/ or, esp. Brit., /ee veuh /, n. 1. any process of formation or growth; development: the evolution of a language; the evolution of the airplane. 2. a product of such development; something… …   Universalium

  • Hardy–Weinberg principle — for two alleles: the horizontal axis shows the two allele frequencies p and q and the vertical axis shows the genotype frequencies. Each graph shows one of the three possible genotypes. The Hardy–Weinberg principle (also known by a variety of… …   Wikipedia

  • Natural selection — For other uses, see Natural Selection (disambiguation). Part of a series on Evolutionary Biology …   Wikipedia

  • literature — /lit euhr euh cheuhr, choor , li treuh /, n. 1. writings in which expression and form, in connection with ideas of permanent and universal interest, are characteristic or essential features, as poetry, novels, history, biography, and essays. 2.… …   Universalium

  • Кимура, Мото — Запрос «Кимура» перенаправляется сюда. Cм. также другие значения. Мотоо Кимура яп. 木村資生 Дата рождения: 13 ноября 1924 Место рождения …   Википедия

  • Кимура, Мотоо — Запрос «Кимура» перенаправляется сюда; см. также другие значения. Мотоо Кимура яп. 木村資生 Дата рождения: 13 ноября 1924 …   Википедия

  • Кимура М — Запрос «Кимура» перенаправляется сюда. Cм. также другие значения. Мотоо Кимура яп. 木村資生 Дата рождения: 13 ноября 1924 Место рождения …   Википедия

  • Кимура М. — Запрос «Кимура» перенаправляется сюда. Cм. также другие значения. Мотоо Кимура яп. 木村資生 Дата рождения: 13 ноября 1924 Место рождения …   Википедия

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”