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Human genetic variation is the natural variation in gene frequencies observed between the genomes of individuals or groups of humans. The human genome is the Genome of Homo sapiens, which is stored on 23 chromosome pairs Variation occurs at both the individual level (differences between individual people) and at the geographic level, i. e. differences between groups of people living in different parts of the world (ethnic groups, races). Notions of race based on Human genetic variation have replaced historical approaches such as Craniology with the advent of Human genetics in

In genetics there may be multiple variants of any given gene (polymorphism), these are called alleles. Polymorphism in biology occurs when two or more clearly different Phenotypes exist in the same population of a species — in other words the occurrence of more than one An allele (ˈæliːl (UK /əˈliːl/ (US (from the Greek αλληλος allelos, meaning each other) is one member of a pair or series of different forms Any individual human has only two copies of any given allele, one inherited from their mother and the other from their father, but many more different versions of the gene may exist. Genetic variation is also distributed geographically, the frequency of any given allele may be greater in humans from on geographic region than in humans from some other region.

There are at least two reasons why genetic variation is geographically distributed:

The study of human geographic variation has both evolutionary significance and medical applications. The study can help scientists understand ancient human population migrations as well as how different human groups are biologically related to one another. From a medical perspective the study of human genetic variation may be important because some disease causing alleles occur at a greater frequency in people from specific geographic regions.

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Distribution of variation

The differences in many patterns of genetic variation between humans and other species awaits additional genetic studies of human populations and nonhuman species. But the data gathered to date suggest that human variation exhibits several distinctive characteristics. First, compared with many other mammalian species, humans are genetically less diverse—a counterintuitive finding, given our large population and worldwide distribution (Li and Sadler 1991; Kaessmann et al. 2001). For example, the chimpanzee subspecies living just in central and western Africa have higher levels of diversity than do humans (Ebersberger et al. 2002; Yu et al. 2003; Fischer et al. 2004).

Two random humans are expected to differ at approximately 1 in 1000 nucleotides, whereas two random chimpanzees differ at 1 in 500 nucleotide pairs. Nucleotides are Organic compounds that consist of three joined structures a nitrogenous base a Sugar, and a Phosphate group However, with a genome of approximate 3 billion nucleotides, on average two humans differ at approximately 3 million nucleotides. Most of these single nucleotide polymorphisms (SNPs) are neutral, but some are functional and influence the phenotypic differences between humans. A single nucleotide polymorphism ( SNP, pronounced snip) is a DNA sequence variation occurring when a single Nucleotide - A, T The neutral theory of molecular evolution is an influential theory that was introduced with provocative effect by Motoo Kimura in the late 1960s and early 1970s It is estimated that about 10 million SNPs exist in human populations, where the rarer SNP allele has a frequency of at least 1% (see International HapMap Project). The International HapMap Project is an organization whose goal is to develop a Haplotype map of the Human genome (the HapMap) which will describe the common

The distribution of variants within and among human populations also differs from that of many other species. The details of this distribution are impossible to describe succinctly because of the difficulty of defining a "population," the clinal nature of variation, and heterogeneity across the genome (Long and Kittles 2003). In general, however, 5%–15% of genetic variation occurs between large groups living on different continents, with the remaining majority of the variation occurring within such groups (Lewontin 1972; Jorde et al. 2000a; Hinds et al. 2005). This distribution of genetic variation differs from the pattern seen in many other mammalian species, for which existing data suggest greater differentiation between groups (Templeton 1998; Kittles and Weiss 2003).

Our history as a species also has left genetic signals in regional populations. For example, in addition to having higher levels of genetic diversity, populations in Africa tend to have lower amounts of linkage disequilibrium than do populations outside Africa, partly because of the larger size of human populations in Africa over the course of human history and partly because the number of modern humans who left Africa to colonize the rest of the world appears to have been relatively low (Gabriel et al. In Population genetics, linkage disequilibrium is the non-random association of Alleles at two or more loci, not necessarily on the same Chromosome 2002). In contrast, populations that have undergone dramatic size reductions or rapid expansions in the past and populations formed by the mixture of previously separate ancestral groups can have unusually high levels of linkage disequilibrium (Nordborg and Tavare 2002).

In the field of population genetics, it is believed that the distribution of neutral polymorphisms among contemporary humans reflects human demographic history. Population genetics is the study of the Allele frequency distribution and change under the influence of the four evolutionary forces Natural selection, Genetic Polymorphism in biology occurs when two or more clearly different Phenotypes exist in the same population of a species — in other words the occurrence of more than one It is believed that humans passed through a population bottleneck before a rapid expansion coinciding with migrations out of Africa leading to an African-Eurasian divergence around 100,000 years ago (ca. A population bottleneck (or genetic bottleneck) is an Evolutionary event in which a significant percentage of a population or species is killed or otherwise prevented In Paleoanthropology, the recent African origin of modern humans is one of two hypotheses of the origin of anatomically modern humans Homo sapiens sapiens 5,000 generations), followed by a European-Asian divergence about 40,000 years ago (ca. 2,000 generations). Richard G. Klein, Nicholas Wade and Spencer Wells, among others, have postulated that modern humans did not leave Africa and successfully colonize the rest of the world until as recently as 60,000 - 50,000 years B. Richard G Klein (born April 11, 1941) is a Professor of Anthropological Sciences at Stanford University. Nicholas Wade is a British-born scientific reporter editor and author who currently writes for the Science Times section of The New York Times. Spencer Wells (born April 6 1969 in Georgia United States) is a Geneticist and Anthropologist, and an Explorer-in-Residence at the P. , pushing back the dates for subsequent population splits as well.

The rapid expansion of a previously small population has two important effects on the distribution of genetic variation. Populations with small population size behave differently to larger populations First, the so-called founder effect occurs when founder populations bring only a subset of the genetic variation from their ancestral population. In Population genetics, the founder effect refers to the loss of genetic variation when a new colony is established by a very small number of individuals from a larger Second, as founders become more geographically separated, the probability that two individuals from different founder populations will mate becomes smaller. The effect of this assortative mating is to reduce gene flow between geographical groups, and to increase the genetic distance between groups. Assortative mating (also called assortative pairing) takes place when sexually reproducing Organisms tend to Mate with individuals that are like The expansion of humans from Africa affected the distribution of genetic variation in two other ways. First, smaller (founder) populations experience greater genetic drift because of increased fluctuations in neutral polymorphisms. In Population genetics, genetic drift is the accumulation of random events that change the makeup of a gene pool slightly but often compound over time Second, new polymorphisms that arose in one group were less likely to be transmitted to other groups as gene flow was restricted.

Many other geographic, climatic, and historical factors have contributed to the patterns of human genetic variation seen in the world today. For example, population processes associated with colonization, periods of geographic isolation, socially reinforced endogamy, and natural selection all have affected allele frequencies in certain populations (Jorde et al. 2000b; Bamshad and Wooding 2003). In general, however, the recency of our common ancestry and continual gene flow among human groups have limited genetic differentiation in our species.

Substructure in the human population

Triangle plot shows average admixture of five North American ethnic groups.[citation needed] Individuals that self-identify with each group can be found at many locations on the map, but on average groups tend to cluster differently.
Triangle plot shows average admixture of five North American ethnic groups. Individuals that self-identify with each group can be found at many locations on the map, but on average groups tend to cluster differently.

New data on human genetic variation has reignited the debate surrounding race. Most of the controversy surrounds the question of how to interpret this new data, and whether conclusions based on existing data are sound. A large majority of researchers endorse the view that continental groups do not constitute different subspecies. However, other researchers still debate whether evolutionary lineages should rightly be called "races". These questions are particularly pressing for biomedicine, where self-described race is often used as an indicator of ancestry (see race in biomedicine below). Biomedicine, also known as theoretical medicine, is a term that comprises the knowledge and research which is more or less in common to the fields of human medicine Race and health research is mostly from the United States. It has found both current and historical racial differences in the frequency treatments and availability of treatments

Although the genetic differences among human groups are relatively small, these differences in certain genes such as duffy, ABCC11, SLC24A5, called ancestry-informative markers (AIMs) nevertheless can be used to reliably situate many individuals within broad, geographically based groupings or self-identified race. Earwax, also known by the Medical term cerumen, is a yellowish waxy substance secreted in the Ear canal of Humans and many other SLC24A5 ( solute carrier family 24 member 5) is a Gene that is thought to explain between 25 and 38% of Skin pigmentation variation between Black ancestry-informative marker (AIM is a set of polymorphisms for a locus generally from humans which exhibits substantially different frequencies between populations from different geographical For example, computer analyses of hundreds of polymorphic loci sampled in globally distributed populations have revealed the existence of genetic clustering that roughly is associated with groups that historically have occupied large continental and subcontinental regions (Rosenberg et al. 2002; Bamshad et al. 2003).

Some commentators have argued that these patterns of variation provide a biological justification for the use of traditional racial categories. They argue that the continental clusterings correspond roughly with the division of human beings into sub-Saharan Africans; Europeans, Western Asians, Southern Asians and Northern Africans + Eastern Asians, Southeast Asians, Polynesians and Native Americans; and other inhabitants of Oceania (Melanesians, Micronesians & Australian Aborigines) (Risch et al. 2002). Other observers disagree, saying that the same data undercut traditional notions of racial groups (King and Motulsky 2002; Calafell 2003; Tishkoff and Kidd 2004). They point out, for example, that major populations considered races or subgroups within races do not necessarily form their own clusters. Thus, samples taken from India and Pakistan affiliate with Europeans or eastern Asians rather than separating into a distinct cluster.

Furthermore, because human genetic variation is clinal, many individuals affiliate with two or more continental groups. Thus, the genetically based "biogeographical ancestry" assigned to any given person generally will be broadly distributed and will be accompanied by sizable uncertainties (Pfaff et al. 2004).

In many parts of the world, groups have mixed in such a way that many individuals have relatively recent ancestors from widely separated regions. Although genetic analyses of large numbers of loci can produce estimates of the percentage of a person's ancestors coming from various continental populations (Shriver et al. 2003; Bamshad et al. 2004), these estimates may assume a false distinctiveness of the parental populations, since human groups have exchanged mates from local to continental scales throughout history (Cavalli-Sforza et al. 1994; Hoerder 2002). Even with large numbers of markers, information for estimating admixture proportions of individuals or groups is limited, and estimates typically will have wide confidence intervals or CIs (Pfaff et al. In Statistics, a confidence interval (CI is an interval estimate of a Population parameter. 2004).

Variation in phenotype

Further information: skin color, hair color, eye color, body hair, Human height, Human weight, and human intelligence
Further information: Height and intelligence, race and intelligence, race and height

The distribution of many physical traits resembles the distribution of genetic variation within and between human populations (American Association of Physical Anthropologists 1996; Keita and Kittles 1997). Human skin color can range from almost black (due to very high concentrations of the dark brown pigment melanin to nearly colorless (appearing reddish white due to the Blood Hair color is the pigmentation of hair follicles due to the two types of melanin Eumelanin and Phaeomelanin. Eye color is a Polygenic trait and is determined by the amount and type of Pigments in the Eye 's iris. Androgenic hair, colloquially Body hair, is the Terminal hair on the Human body developed during and after Puberty. Human height varies according to both "nature" and "nurture". A positive Correlation exists between human IQ and height within national populations The study of race and intelligence is a controversial field which seeks to determine whether or not human intellectual abilities vary between races The modern controversy Human height varies according to both "nature" and "nurture". The American Association of Physical Anthropologists (AAPA is an American -based International Scientific society of Physical anthropologists For example, ~90% of the variation in human head shapes occurs within continental groups, and ~10% separates groups, with a greater variability of head shape among individuals with recent African ancestors (Relethford 2002).

Variation in a trait under selection, skin colour

A prominent exception to the common distribution of physical characteristics within and among groups is skin color. Approximately 10% of the variance in skin color occurs within groups, and ~90% occurs between groups (Relethford 2002). This distribution of skin color and its geographic patterning — with people whose ancestors lived predominantly near the equator having darker skin than those with ancestors who lived predominantly in higher latitudes — indicate that this attribute has been under strong selective pressure. Any cause that reduces reproductive success in a proportion of a population potentially exerts Evolutionary pressure or selection pressure. Darker skin appears to be strongly selected for in equatorial regions to prevent sunburn, skin cancer, the photolysis of folate, and damage to sweat glands (Sturm et al. 2001; Rees 2003). A leading hypothesis for the selection of lighter skin in higher latitudes is that it enables the body to form greater amounts of vitamin D, which helps prevent rickets (Jablonski 2004). Evidence for this includes the finding that a substantial portion of the differences of skin color between Europeans and Africans resides in a single gene, SLC24A5 the threonine-111 allele of which was found in 98. SLC24A5 ( solute carrier family 24 member 5) is a Gene that is thought to explain between 25 and 38% of Skin pigmentation variation between Black 7 to 100% among several European samples, while the alanine-111 form was found in 93 to 100% of samples of Africans, East Asians and Indigenous Americans (Lamason et al. 2005). However, the vitamin D hypothesis is not universally accepted (Aoki 2002), and lighter skin in high latitudes may correspond simply to an absence of selection for dark skin (Harding et al. 2000). Melanin which serves as the pigment, is located in the epidermis of the skin, and is based on hereditary gene expression. Melanin is a class of compounds found in the Plant, Animal and Protista kingdoms, where it serves predominantly as a Pigment. Epidermis is the outermost layer of the Skin. It forms the waterproof protective wrap over the body's surface and is made up of stratified squamous Epithelium with Gene expression is the process by which inheritable information from a Gene, such as the DNA sequence, is made into a functional Gene product, such

Because skin color has been under strong selective pressure, similar skin colors can result from convergent adaptation rather than from genetic relatedness. Sub-Saharan Africans, tribal populations from southern India, and Indigenous Australians have similar skin pigmentation, but genetically they are no more similar than are other widely separated groups. Furthermore, in some parts of the world in which people from different regions have mixed extensively, the connection between skin color and ancestry has been substantially weakened (Parra et al. 2004). In Brazil, for example, skin color is not closely associated with the percentage of recent African ancestors a person has, as estimated from an analysis of genetic variants differing in frequency among continent groups (Parra et al. 2003).

Considerable speculation has surrounded the possible adaptive value of other physical features characteristic of groups, such as the constellation of facial features observed in many eastern and northeastern Asians (Guthrie 1996). However, any given physical characteristic generally is found in multiple groups (Lahr 1996), and demonstrating that environmental selective pressures shaped specific physical features will be difficult, since such features may have resulted from sexual selection for individuals with certain appearances or from genetic drift (Roseman 2004).

References

September 1998, Vol. 100, No. 3, pp. 632-650 [7]

External links

See also

Notions of race based on Human genetic variation have replaced historical approaches such as Craniology with the advent of Human genetics in Archaeogenetics, a term coined by Colin Renfrew, refers to the application of the techniques of molecular Population genetics to the study of the Human Human evolutionary genetics studies how one Human genome differs from the other the evolutionary past that gave rise to it and its current effects In paleoanthropology, the multiregional hypothesis is one of two accounts of the origin of anatomically modern humans Homo sapiens. In Paleoanthropology, the recent African origin of modern humans is one of two hypotheses of the origin of anatomically modern humans Homo sapiens sapiens The Hybrid -origin hypothesis of Human origins argues that all or at least some of the Genetic variation between the contemporary human races is attributable European populations have a complicated demographic and genetic history including many layers of successive migrations between different time periods The Genetics and Archaeogenetics of the ethnic groups '''of''' South Asia aim at uncovering these groups' Genetic history The 1000 Genomes Project, launched in January 2008 is an international research effort to establish by far the most detailed catalogue of Human genetic variation.
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