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Lettris is a curious tetris-clone game where all the bricks have the same square shape but different content. Each square carries a letter. To make squares disappear and save space for other squares you have to assemble English words (left, right, up, down) from the falling squares.
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Race and genetics
The relationship between race and genetics has relevance for the ongoing controversies regarding race.
Ongoing genetic research has investigated how ancestral human populations migrated in the ancestral geographic environment into different geographic areas. Today it is possible to determine, by genetic analysis, the geographic ancestry of a person and the degree of ancestry from each region. Such analyses can pinpoint the migrational history of a person's ancestors with a high degree of accuracy. Often, due to practices of group endogamy, allele frequencies cluster locally around kin groups and lineages, or by national, cultural or linguistic boundaries - giving a detailed degree of correlation between genetic clusters and population groups when considering many alleles simultaneously.
However, biological variation of any single human genetic trait is often described best as clinal, with gradual transitions of trait frequencies between different population clusters. Furthermore, different clines don't align around the same centers, constructing a much more complex picture of variation than merely large continental groupings. In the past, like today, all populations were genetically heterogeneous with larger degrees of variation within the groups than between them. This means that concepts of "pure" and "mixed" ancestry are problematic to work with.
For these reasons the fact that there are genetically differentiated populations does not necessarily validate the concept of race - since racial categorization of individuals is generally based on very broad criteria founded on arbitrary phenotypical characteristics such as skin color, which do not correlate well with geographic ancestry. For example, there is no logical reason that the typical American racial classification of people in groups of Black and White skin and Asian and Amerindian geographical ancestry is a better description of biological variation than other possible groupings of traits. Specialists in race, both geneticists and anthropologists, maintain that modern ideas of race are therefore primarily historical constructions that reflect the pattern of contact between previously distinct populations in the colonial period.
Some anthropologists, particularly those working with forensics, do consider race to be a valid biological category because it is often possible to determine the racial category of a person by examining physical remains. Others note that what is really identified is the geographical phenotype[clarification needed] of the remains which can then be used to infer how the individual would have been classified racially according to the local social construction of racial categories.
The human lineage diverged from the common ancestor with chimpanzees about 5–7 million years ago. The genus Homo evolved by about 2.3 to 2.4 million years ago from Australopithecines. Several species and subspecies of Homo evolved and are now extinct. These include Homo erectus, which inhabited Asia, and Homo sapiens neanderthalensis, which inhabited West Eurasia. Archaic Homo sapiens evolved between 400,000 and 250,000 years ago.
The dominant view among scientists concerning the origin of anatomically modern humans is the "Out of Africa" or recent African origin hypothesis, which argues that Homo sapiens arose in Africa and migrated out of the continent around 50,000 to 100,000 years ago, replacing populations of Homo erectus in Asia and Homo neanderthalensis in Europe. An alternative multiregional hypothesis argue that Homo sapiens evolved as geographically separate but interbreeding populations stemming from a worldwide migration of Homo erectus out of Africa nearly 2.5 million years ago. This theory has been challenged by the other "Out of Africa" theory, although it has recently been shown with DNA evidence that non Homo sapiens Neanderthal genomes may have contributed about 4% of non-African heredity, and the recently discovered Denisova hominin may have contributed 6% of the genome of Melanesians.
Genetic variation comes from mutations in genetic material, migration between populations (gene flow), and the reshuffling of genes through sexual reproduction. The two main mechanisms that produce evolution are natural selection and genetic drift. A special case of genetic drift is the founder effect. Epigenetic inheritance are heritable changes in phenotype (appearance) or gene expression caused by mechanisms other than changes in the underlying DNA sequence.
Many human phenotypes are polygenic, meaning that they depend on the interaction among many genes. Polygeneity makes the study of individual phenotypic differences more difficult. Additionally, phenotypes may be influenced by environment as well as by genetics. The measure of the genetic role in phenotypes is heritability.
Nucleotide diversity is based on single mutations called single nucleotide polymorphisms (SNPs). The nucleotide diversity between humans is about 0.1%, which is 1 difference per 1,000 nucleotides between two humans chosen at random. This amounts to approximately 3 million SNPs since the human genome has about 3 billion nucleotides. It is estimated that a total of 10 million SNPs exist in the human population.
Recent analysis has shown that non-SNP variation accounts for much more human genetic variation than single nucleotide diversity. This non-SNP variation includes copy number variation and results from deletions, inversions, insertions and duplications. It is estimated that approximately 0.4% of the genomes of unrelated people typically differ with respect to copy number. When copy number variation is included, human to human genetic variation is estimated to be at least 0.5%.
The earliest classification attempts were done using surface traits such as done in anthropometry. This is argued to have caused problems for early anthropologists whose simplistic approach was inadequate for classifying race based on visible traits.
Prior to the discovery of DNA as the hereditary material, scientists used blood proteins (the human blood group systems) to study human genetic variation. Research by Ludwik and Hanka Herschfeld during World War I found that the frequencies of blood groups A and B differed greatly from region to region. For example, among Europeans, 15% were group B and 40% were group A. Eastern Europeans and Russians had higher frequencies of group B, with people from India having the highest proportion. The Herschfelds concluded that humans were made of two different "biochemical races," each with its own origin. It was hypothesized that these two pure races later became mixed, resulting in the complex pattern of groups A and B. This was one of the first theories of racial differences to include the idea that visible human variation did not necessarily correlate with invisible genetic variation. It was expected that groups that had similar proportions of the blood groups would be more closely related in racial terms, but instead it was often found that groups separated by large distances, such as those from Madagascar and Russia, had similar frequencies. This confounded scientists who were attempting to learn more about human evolutionary history.
Today researchers often use direct genetic testing. Unlike earlier research using one or a few traits or proteins, today this often involve the simultaneous study of hundreds or thousands of genetic markers or even the whole genome.
Multi-Locus Allele Clusters
There are several mathematical methods for examining if a population have more or less distinct genetic subgroups and to quantify this. Many genetic markers from many individuals are examined simultaneously in order to find the population genetic structure. The basic idea is that while such subgroups are not distinct and overlap if looking at the distribution of the variants of one marker only, when many markers are examined simultaneously, then the different subgroups have distinctly different average genetic structure. An individual need not have exactly this average genetic structure and may be described as belonging, to varying degrees, to several subgroups. Such subgroups may be more or less distinct depending on how close a subgroup distribution is to the average genetic structure of the subgroup and how much overlap there are with the distributions of different subgroups. One such mathematical method is cluster analysis. Another is principal components analysis. The population genetic structure found is often similar.
In cluster analysis the number of clusters to search for ("K") is determined in advance; how distinct these clusters are from one another vary. The results obtained by clustering analyses are dependent on several factors:
Genetic distance refers to the genetic divergence between species or between populations within a species. Smaller genetic distances indicate a close genetic relationship whereas large genetic distances indicate a more distant genetic relationship. Genetic distance can be used to compare the genetic similarity between different species, such as humans and chimpanzees. Within a species genetic distance can be used to measure the divergence between different subgroups.
Genetic distance significantly correlates to geographic distance between populations, a phenomenon referred to as "isolation by distance". Genetic distance can also be the result of physical boundaries which naturally restrict gene flow, such as islands, deserts, mountain ranges or dense forests.
Genetic distance is often measured by Fixation index (FST). FST is simply the correlation of randomly chosen alleles within the same sub-population relative to that found in the entire population. It is often expressed as the proportion of genetic diversity due to allele frequency differences among populations. This comparison of genetic variability within and between populations is frequently used in the field of population genetics. The values range from 0 to 1. A zero value implies complete panmixis, that the two populations are interbreeding freely. A value of one would imply the two populations are completely separate.
Cavalli-Sforza has described two major methods of ancestry analysis. Note that current population genetic structure does not necessarily imply that the different current clusters/components found correspond to only one ancestral home per group. One example being a genetic cluster in the US corresponding to Hispanics who have European, Native American, and African ancestries.
Geographic analyses attempt to identify the places of origin, relative importance, and the possible causes involved in the spread of genetic variation over an area. The results can be presented as maps showing how genes vary between populations. Cavalli-Sforza and colleagues have argued that if variations in many genes between populations are investigated simultaneously, they often correspond to population migrations due to, for example, new sources of food, improved transportation, or shifts in political power. For example, in Europe the single most significant direction of genetic variation corresponds to the spread of farming from the Middle East to Europe between 10,000 and 6,000 years ago. Such geographic analysis works best when describing the situation before recent large scale and fast migrations with intermixing of many populations far from their ancestral homes.
Historic analyses use differences in genetic variation, genetic distance being one way to measure this, as a molecular clock indicating the evolutionary relatedness of various species or groups. This method can be used to create evolutionary trees which attempt to reconstruct population separations over time,
Cavalli-Sforza and colleagues have argued that there is a strong correspondence between the language families found in linguistic research and the populations and the tree they found in their 1994 study. As a general rule, there is shorter genetic distances between populations using languages from the same language family. The notable exceptions to this rule are Sami, Tibetans, and Ethiopians, who are genetically associated with populations which speak languages belonging to different language families. For example, the Sami speak an Uralic language yet are according to the genetic analysis mainly Europeans. This is argued to have resulted from migration and interbreeding with Europeans while retaining the original language. There are similar explanations for the other exceptions. There is also a high agreement between dates from research done in archeology and as calculated using genetic distance.
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Modern definitions of what constitute the distinct races of humanity are still rooted in taxonomic classifications first developed in 18th and 19th century Europe, before the later availability of many tools of modern science, in particular population genetics (see descriptions in the article Race (classification of humans). Part of the discussion surrounding the definition of "race" among human beings has overlapped in the past with debates regarding what constitutes a distinct species, known as the "species problem".
Since the 1960's a majority of scientists have come to understand the concept of race as a social construct mapped on to phenotypes in different culturally determined ways, and not as a biological concept. A 2000 study by Celera Genomics found that human beings largely have similar genetic input, and that citizens of any given village in the world, whether in Scotland or Tanzania, hold 90 percent of the genetic variability that humanity has to offer. The study found only .01% of genes account for a person's external appearance. Biological adaptation also plays a role in phenotype of bodily features and skin type. According to Luigi Luca Cavalli-Sforza, "Skin color and body size are less subject to genetic influence since they are also affected by exposure to the sun and diet, but there is always a hereditary component that can be quite important."
The research techniques can be used to detect subtle genetic population differences if enough genetic markers are used. One example being that the East Asian populations Japanese and Chinese can be identified if enough markers are used. Sub-Saharan Africans have higher genetic diversity than other populations which may be a problem to seeing them as a single race.
In 1972 Richard Lewontin performed a FST statistical analysis using 17 markers including blood group proteins. His results were that the majority of genetic differences between humans, 85.4%, were found within a population, 8.3% of genetic differences were found between populations within a race, and only 6.3% was found to differentiate races which in the study were Caucasian, African, Mongoloid, South Asian Aborigines, Amerinds, Oceanians, and Australian Aborigines. Since then, other analyses have found FST values of 6%-10% between continental human groups, 5-15% between different populations occupying the same continent, and 75-85% within populations. Lewontin's argument led a number of authors publishing in the 1990s and 2000s to follow Lewontin's verdict that race is biologically a meaningless concept.
While acknowledging the correctness of Lewontin's observation that racial groups are genetically homogeneous, geneticist A. W. F. Edwards in the paper "Human Genetic Diversity: Lewontin's Fallacy" (2003) argued that the conclusion that racial groups can not be genetically distinguished from each other is incorrect. Edwards argued that when multiple allelles are taken into account genetic differences do tend to cluster in geographic patterns roughly corresponding to the groups commonly defined as races. This is because most of the information that distinguishes populations from each other is hidden in the correlation structure of allele frequencies, making it possible to highly reliably classify individuals using the mathematical techniques described above. Edwards argued that, even if the probability of misclassifying an individual based on a single genetic marker is as high as 30% (as Lewontin reported in 1972), the misclassification probability becomes close to zero if enough genetic markers are studied simultaneously. Edwards saw Lewontin's argument as being based mostly in a political stance that denies the existence biological difference in order to argue for social equality. 
Richard Dawkins (2005) agreed with Edwards' view, summarizing the argument against Lewontin as being, "However small the racial partition of the total variation may be, if such racial characteristics as there are highly correlate with other racial characteristics, they are by definition informative, and therefore of taxonomic significance."
Alan Templeton (2003) argued that in the nonhuman literature an FST of at least 25%-30% is a standard criterion for the identification of a subspecies. John Goodrum has noted that Templeton incorrectly cited this figure from a 1997 article from Herpetological Review entitled "Subspecies and Classification". The authors of that paper do not refer to Fst values. They are referring the 75 percent rule for subspecies.
Henry Harpending (2002) has argued that the magnitude of human FST values imply that "kinship between two individuals of the same human population is equivalent to kinship between grandparent and grandchild or between half siblings. The widespread assertion that this is small and insignificant should be reexamined." 
Sarich and Miele (2004) have argued that estimates of genetic difference between individuals of different populations fail to take into account human diploidity. "The point is that we are diploid organisms, getting one set of chromosomes from one parent and a second from the other. To the extent that your mother and father are not especially closely related, then, those two sets of chromosomes will come close to being a random sample of the chromosomes in your population. And the sets present in some randomly chosen member of yours will also be about as different from your two sets as they are from one another. So how much of the variability will be distributed where? First is the 15 percent that is interpopulational. The other 85 percent will then split half and half (42.5 percent) between the intra- and interindividual within-population comparisons. The increase in variability in between-population comparisons is thus 15 percent against the 42.5 percent that is between-individual within-population. Thus, 15/42.5 is 32.5 percent, a much more impressive and, more important, more legitimate value than 15 percent."
Anthropologists such as C. Loring Brace and Jonathan Kaplan and geneticist Joseph Graves, have argued that while there it is certainly possible to find biological and genetic variation that corresponds roughly to the groupings normally defined as races, this is true for almost all geographically distinct populations. The cluster structure of the genetic data is therefore dependent on the initial hypotheses of the researcher and the populations sampled. When one samples continental groups the clusters become continental, if one had chosen other sampling patterns the clusters would be different. Weiss and Fullerton have noted that if one sampled only Icelanders, Mayans and Maoris, three distinct clusters would form and all other populations could be described as being composed of admixtures of Maori, Icelandic and Mayan genetic materials. Kaplan therefore argues that seen in this way both Lewontin and Edwards are right in their arguments. He concludes that while racial groups are characterized by different allele frequencies, this does not mean that racial classification is a natural taxonomy of the human species, because multiple other genetic patterns can be found in human populations that crosscut racial distinctions. In this view racial groupings are social constructions that also have biological reality which is largely an artefact of how the category has been constructed.
Jorde and Wooding (2004) wrote that some studies have argued that clusters from genetic markers did not correspond well to the subjects' self-identified race/ethnic group. These studies, however, were based on only several dozen or fewer genetic markers, and such a number, unless carefully selected, are argued to not be sufficient. In contrast, studies based on more genetic markers have found high agreements.
A study by Tang et al. in 2005 used 326 genetic markers in order to determine genetic clusters. The 3,636 subjects involved in the study, from the United States and Taiwan, self-identified as belonging to white, African American, East Asian, or Hispanic (=self-identified race/ethnic group (SIRE)). The study found "nearly perfect correspondence between genetic cluster and SIRE for major ethnic groups living in the United States, with a discrepancy rate of only 0.14%."
Paschou et al. (2010) found "essentially perfect" agreement between 51 self-reported populations of origin and the population genetic structure found using 650,000 genetic markers. Selecting for especially informative genetic makers allowed a reduction to less than 650 while still retaining close to 100% accuracy.
That there is correspondence between genetic clusters in a current population, such as the current US population, and self-identified race/ethnic groups does not necessarily mean that such a cluster/group corresponds to only one ancestral origin/population. African Americans have an estimated 10%–20% European admixture. The Hispanic group have European, Native American, and African ancestries. In Brazil, there has been extensive admixture between Europeans, Amerindians, and Africans resulting in no clear discontinuities in skin color in the population and relatively weak associations between between self-reported race (called Color in Brazil probably because it captures the continuous aspects) and African ancestry as well as between objectively measured skin color and African ancestry.
One argument is that genetic distances on average increase in a continuous manner with geographic distance, which causes any threshold or dividing line to be arbitrary. Any two neighboring villages or towns will show some genetic differentiation from each other and thus could be defined as a race. Thus any attempt to classify races would be imposing an artificial discontinuity on what is otherwise a naturally occurring continuous phenomenon. This has been argued to explain why some studies on population genetic structure have yielded varying results depending on the methodology used.
Ring species show that also a continuous change in genetic variation can produce very large differences between different populations in a species.
Rosenberg et al. (2005) have argued, based on cluster analysis, that populations do not always vary continuously and that the population genetic structure is consistent if enough genetic markers and subjects are included. "Examination of the relationship between genetic and geographic distance supports a view in which the clusters arise not as an artifact of the sampling scheme, but from small discontinuous jumps in genetic distance for most population pairs on opposite sides of geographic barriers, in comparison with genetic distance for pairs on the same side. Thus, analysis of the 993-locus dataset corroborates our earlier results: if enough markers are used with a sufficiently large worldwide sample, individuals can be partitioned into genetic clusters that match major geographic subdivisions of the globe, with some individuals from intermediate geographic locations having mixed membership in the clusters that correspond to neighboring regions." They also wrote, regarding a model with five clusters corresponding to Africa, Eurasia (Europe, Middle East, and Central/South Asia), East Asia, Oceania, and the Americas, that "For population pairs from the same cluster, as geographic distance increases, genetic distance increases in a linear manner, consistent with a clinal population structure. However, for pairs from different clusters, genetic distance is generally larger than that between intracluster pairs that have the same geographic distance. For example, genetic distances for population pairs with one population in Eurasia and the other in East Asia are greater than those for pairs at equivalent geographic distance within Eurasia or within East Asia. Loosely speaking, it is these small discontinuous jumps in genetic distance—across oceans, the Himalayas, and the Sahara—that provide the basis for the ability of STRUCTURE to identify clusters that correspond to geographic regions."
The above discussion applies to populations in their ancestral homes when migrations and gene flow were slow. Recent large and fast migrations due to changed technology have changed this. Thus, regarding the situation today in the United States, Tang et al. (2004) write that "we detected only modest genetic differentiation between different current geographic locales within each race/ethnicity group. Thus, ancient geographic ancestry, which is highly correlated with self-identified race/ethnicity—as opposed to current residence—is the major determinant of genetic structure in the U.S. population."
Cluster analysis has been criticized for that number of clusters to search for are decided in advance with many different values possible although with varying probability. Principal components analysis does not decide the numbers of components to search for in advance. An increasing number of studies have used it in recent years.
While knowing a persons race can be helpful in some situations in medicine, it has been argued that this is of limited value since also persons from the same race vary from one another.
Witherspoon et al. (2007) have argued that even when individuals can be reliably assigned to specific population groups, it may still be possible for two randomly chosen individuals from different populations/clusters to be more similar to each other than to a randomly chosen member of their own cluster. They found that many thousands of genetic markers had to be used in order for the answer to the question "How often is a pair of individuals from one population genetically more dissimilar than two individuals chosen from two different populations?" to be "never". This assumed three population groups separated by large geographic ranges (European, African and East Asian). The entire world population is much more complex and studying an increasing number of groups would require an increasing number of markers for the same answer. Witherspoon et al. conclude that "caution should be used when using geographic or genetic ancestry to make inferences about individual phenotypes." Witherspoon et al. concluded that, "The fact that, given enough genetic data, individuals can be correctly assigned to their populations of origin is compatible with the observation that most human genetic variation is found within populations, not between them. It is also compatible with our ﬁnding that, even when the most distinct populations are considered and hundreds of loci are used, individuals are frequently more similar to members of other populations than to members of their own population."
This is somewhat similar to a realization by anthropologist Norman Sauer in a 1992 article on the ability of forensic anthropologists to be able to assign a "race" to a skeleton based on its craniofacial features and limb morphology. He concluded that "the successful assignment of race to a skeletal specimen is not a vindication of the race concept, but rather a prediction that an individual, while alive was assigned to a particular socially constructed ‘racial’ category. A specimen may display features that point to African ancestry. In this country that person is likely to have been labeled Black regardless of whether or not such a race actually exists in nature.
Human skin color vary for different populations. The leading explanation is that skin colour adapts to sunlight intensities which produce vitamin D deficiency or ultraviolet light damage to folic acid. Other hypotheses include protection from ambient temperature, infections, skin cancer or frostbite, an alteration in food, and sexual selection. The gene that causes light skin color in Europeans is different from the gene that causes light skin in East Asians. Europeans have a different version of the SLC24A5 than East Asians possibly indicating that they evolved light skin independently.
The most widely used human racial categories are based on various combinations of visible traits such as skin color, eye shape and hair texture. However, some argue that many of these traits are non-concordant in that they are not necessarily expressed together. For example, skin color and hair texture vary independently. Some examples of non-concordance include:
Others argue that this is just an example of Lewontin's Fallacy. On the contrary, if several traits are looked at the same time, then today forensic anthropologists can classify a person's race with an accuracy close to 100% based on only skeletal remains.
A 2010 examination of 18 widely used English anatomy textbooks found that every one relied on the race concept. The study gives examples of how the textbooks claim that anatomical features vary between races.
Neil Risch states that numerous studies over past decades have documented biological differences among the races with regard to susceptibility and natural history of chronic diseases. Genes may be under strong selection in response to local diseases. For example, people who are duffy negative tend to have higher resistance to malaria. Most Africans are duffy negative and most non-Africans are duffy positive. A number of genetic diseases more prevalent in malaria-afflicted areas may provide some genetic resistance to malaria including sickle cell disease, thalassaemias, glucose-6-phosphate dehydrogenase, and possibly others. Cystic fibrosis is the most common life-limiting autosomal recessive disease among people of European heritage. Numerous hypotheses have suggested that it provides a heterozygote advantage by giving resistance to diseases earlier common in Europe.
Information about a person's population of origin may in some situations help making a diagnosis and adverse drug responses may vary between such groups. Because of the correlation between self-identified race and genetic clusters, medical treatments whose results are influenced by genetics often have varying rates of success between self-defined racial groups. For this reason, some doctors consider a patient’s race while attempting to identify the most effective possible treatment, and some drugs are marketed with race-specific instructions. Jorde and Wooding (2004) have argued that, because of the genetic variation within racial groups, when "it finally becomes feasible and available, individual genetic assessment of relevant genes will probably prove more useful than race in medical decision making." Even so, race will continue to be important when looking at groups instead of individuals such as in epidemiologic research.
Lactose tolerance and alcohol tolerance differ with geographic ancestry in part due to genetic factors. Lactose tolerance appears to be an evolutionarily recent adaptation to dairy consumption, and has occurred independently in both northern Europe and east Africa in populations with a historically pastoral lifestyle.
The overrepresentation of certain ethnicities with respect to certain sports has led some to question whether there is a genetic component giving certain races a competitive advantage. Others point out that such overrepresentations are not necessarily due to genetic causes. Such views differ between nations. Among Chinese, the proposition that there are genetic differences affecting sports performance is a widely accepted. A 1994 examination of 32 English sport/exercise science textbooks found that 7 (21.9%) claimed that there are biophysical differences due to race that might explain differences in sports performance, 24 (75%) did not mention nor refute the concept, and 1 (3.12%) expressed caution with the idea.
There is an ongoing scientific controversy regarding the role of genetics in explaining racial differences in IQ and other measures of intelligence. Some are agnostic about the causes, while others argue that environmental factors explain all of the differences, or that both genetics and environmental factors are important.