Genetic variation is the variation in the genetic material of a population, and includes the nuclear, mitochodrial, ribosomal genomes as well as the genomes of other organelles. New genetic variation is caused by genetic mutation, which may take the form of recombination, migration and/or alterations in the karyotype (the number, shape, size and internal arrangement of the chromosomes). Genetic drift is a statistical measure of the rate of genetic variation in a population. Electron micrograph of a mitochondrion showing its mitochondrial matrix and membranes In cell biology, a mitochondrion (plural mitochondria) (from Greek μιτος or mitos, thread + κουδριον or khondrion, granule) is a membrane-enclosed organelle, found in most eukaryotic cells. ... Figure 1: Ribosome structure indicating small subunit (A) and large subunit (B). ... In biology the genome of an organism is the whole hereditary information of an organism that is encoded in the DNA (or, for some viruses, RNA). ... Schematic of typical animal cell, showing subcellular components. ... In biology, mutations are changes to the base pair sequence of genetic material (either DNA or RNA). ... It has been suggested that chromosomal crossover be merged into this article or section. ... Population genetics is the study of the allele frequency distribution and change under the influence of the four evolutionary forces: natural selection, genetic drift, mutation, and gene flow. ... Karyogram of human male using Giemsa staining. ... Figure 1: A representation of a condensed eukaryotic chromosome, as seen during cell division. ... In population genetics, genetic drift is the statistical effect that results from the influence that chance has on the success of alleles (variants of a gene). ...

For example, all humans are members of the species, Homo sapiens, the first upright mammal, but no two individuals are exactly alike. Even for twins, there are slight differences in their DNA. For the global population, there are many similarities and differences among people. For example, eye color and blood type differ among individual Homo sapiens. Differences in these traits are due to genetic differences, or genetic variation. The human gene pool carries alternative alleles that affect blood type and many other traits. Other species also have variation in their gene pools. For example, apple trees are all members of one species, but the fruit produced by different trees can be red or yellow, hard or soft, sweet or tart, large or small. These differences are caused partly by genetic variation. When two or more alleles of a gene are present in a gene pool, the population is said to be polymorphic. The structure of part of a DNA double helix. ... Blood type is determined, in part, by the ABO blood group antigens present on red blood cells A total of 29 human blood group systems are recognized by the International Society of Blood Transfusion (ISBT). ... The gene pool of a species or a population is the complete set of unique alleles that would be found by inspecting the genetic material of every living member of that species or population. ... Blood type is determined, in part, by the ABO blood group antigens present on red blood cells A total of 29 human blood group systems are recognized by the International Society of Blood Transfusion (ISBT). ... The gene pool of a species or a population is the complete set of unique alleles that would be found by inspecting the genetic material of every living member of that species or population. ... The gene pool of a species or a population is the complete set of unique alleles that would be found by inspecting the genetic material of every living member of that species or population. ...

Population geneticists have studied the gene pools of many species of plants and animals. They have examined variation in obvious traits such as shape and color. In many cases, they have also found genetic variation in the amino-acid sequences of proteins and the nucleotide sequences of DNA. For example, the fruit fly Drosophila melanogaster has an enzyme called alcohol dehydrogenase. There are two slightly different forms of alcohol dehydrogenase that can be distinguished by electrophoresis. The two forms differ by only one amino acid. The amino acid difference is caused by one nucleotide difference in the DNA. In natural populations all over the world, Drosophila melanogaster gene pools are polymorphic for the alcohol dehydrogenase gene. The populations have both types of alleles and produce both varieties of alcohol dehydrogenase. The gene pool of a species or a population is the complete set of unique alleles that would be found by inspecting the genetic material of every living member of that species or population. ... The structure of part of a DNA double helix. ... Alcohol Dehydrogenase Alcohol dehydrogenases are a group of dehydrogenase enzymes that occur in many organisms and facilitate the interconversion between alcohols and aldehydes or ketones. ... Alcohol Dehydrogenase Alcohol dehydrogenases are a group of dehydrogenase enzymes that occur in many organisms and facilitate the interconversion between alcohols and aldehydes or ketones. ... The structure of part of a DNA double helix. ... The gene pool of a species or a population is the complete set of unique alleles that would be found by inspecting the genetic material of every living member of that species or population. ... Alcohol Dehydrogenase Alcohol dehydrogenases are a group of dehydrogenase enzymes that occur in many organisms and facilitate the interconversion between alcohols and aldehydes or ketones. ... Alcohol Dehydrogenase Alcohol dehydrogenases are a group of dehydrogenase enzymes that occur in many organisms and facilitate the interconversion between alcohols and aldehydes or ketones. ...

Genetic Variation Evolution requires genetic variation. If there were no dark moths, the population could not have evolved from mostly light to mostly dark. In order for continuing evolution there must be mechanisms to increase or create genetic variation and mechanisms to decrease it. Mutation is a change in a gene. These changes are the source of new genetic variation. Natural selection operates on this variation. Genetic variation has two components: allelic diversity and non- random associations of alleles. Alleles are different versions of the same gene. For example, humans can have A, B or O alleles that determine one aspect of their blood type. Most animals, including humans, are diploid -- they contain two alleles for every gene at every locus, one inherited from their mother and one inherited from their father. Locus is the location of a gene on a chromosome. Humans can be AA, AB, AO, BB, BO or OO at the blood group locus. If the two alleles at a locus are the same type (for instance two A alleles) the individual would be called homozygous. An individual with two different alleles at a locus (for example, an AB individual) is called heterozygous. At any locus there can be many different alleles in a population, more alleles than any single organism can possess. For example, no single human can have an A, B and an O allele. Considerable variation is present in natural populations. At 45 percent of loci in plants there is more than one allele Introduction to Evolutionary Biology http://www.talkorigins.org/faqs/faq-intro-to-biology.html (2 of 24) [31/8/1999 1:36:43 PM] in the gene pool. [allele: alternate version of a gene (created by mutation)] Any given plant is likely to be heterozygous at about 15 percent of its loci. Levels of genetic variation in animals range from roughly 15% of loci having more than one allele (polymorphic) in birds, to over 50% of loci being polymorphic in insects. Mammals and reptiles are polymorphic at about 20% of their loci - - amphibians and fish are polymorphic at around 30% of their loci. In most populations, there are enough loci and enough different alleles that every individual, identical twins excepted, has a unique combination of alleles. Linkage disequilibrium is a measure of association between alleles of two different genes. [allele: alternate version of a gene] If two alleles were found together in organisms more often than would be expected, the alleles are in linkage disequilibrium. If there two loci in an organism (A and B) and two alleles at each of these loci (A1, A2, B1 and B2) linkage disequilibrium (D) is calculated as D = f(A1B1) * f(A2B2) - f(A1B2) * f(A2B1) (where f(X) is the frequency of X in the population). [Loci (plural of locus): location of a gene on a chromosome] D varies between -1/4 and 1/4; the greater the deviation from zero, the greater the linkage. The sign is simply a consequence of how the alleles are numbered. Linkage disequilibrium can be the result of physical proximity of the genes. Or, it can be maintained by natural selection if some combinations of alleles work better as a team. Natural selection maintains the linkage disequilibrium between color and pattern alleles in Papilio memnon. [linkage disequilibrium: association between alleles at different loci] In this moth species, there is a gene that determines wing morphology. One allele at this locus leads to a moth that has a tail; the other allele codes for a untailed moth. There is another gene that determines if the wing is brightly or darkly colored. There are thus four possible types of moths: brightly colored moths with and without tails, and dark moths with and without tails. All four can be produced when moths are brought into the lab and bred. However, only two of these types of moths are found in the wild: brightly colored moths with tails and darkly colored moths without tails. The non-random association is maintained by natural selection. Bright, tailed moths mimic the pattern of an unpalatable species. The dark morph is cryptic. The other two combinations are neither mimetic nor cryptic and are quickly eaten by birds. Assortative mating causes a non-random distribution of alleles at a single locus. [locus: location of a gene on a chromosome] If there are two alleles (A and a) at a locus with frequencies p and q, the frequency of the three possible genotypes (AA, Aa and aa) will be p2, 2pq and q2, respectively. For example, if the frequency of A is 0.9 and the frequency of a is 0.1, the frequencies of AA, Aa and aa individuals are: 0.81, 0.18 and 0.01. This distribution is called the Hardy-Weinberg equilibrium. Non-random mating results in a deviation from the Hardy-Weinberg distribution. Humans mate assortatively according to race; we are more likely to mate with someone of own race than another. In populations that mate this way, fewer heterozygotes are found than would be predicted under random mating. [heterozygote: an organism that has two different alleles at a locus] A decrease in heterozygotes can be the result of mate choice, or simply the result of population subdivision. Most organisms have a limited dispersal capability, so their mate will be chosen from the local population.

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