Race and genetics

The relationship between race and genetics is relevant to the controversy concerning race classification. In everyday life, many societies classify populations into groups based on phenotypical traits and impressions of probable geographic ancestry and cultural identity—these are the groups usually called "races" in countries like the United States, Brazil, and South Africa. Patterns of variation of human genetic traits are generally clinal, with more abrupt shifts at places where steady gene flow is interrupted. The pattern of genetic variants tend to form larger regional clusters. Such a pattern can be explained by the expansion of the human population from Africa and serial genetic bottlenecks.[1] This causes genetic clusters to correlate statistically with population groups when a number of alleles are evaluated.[2][3][4]

Genetic analysis enables scientists to estimate the geographic ancestry of a person by using ancestry-informative markers, and by inference the probable racial category into which they will be classified in a given society. In that way there is a distinct statistical correlation between gene frequencies and racial categories. However, because all populations are genetically diverse, and because there is a complex relation between ancestry, genetic makeup and phenotype, and because racial categories are based on subjective evaluations of the traits, there is no specific gene that can be used to determine a person's race.[5][6][7]

Some genetic variants that contribute to the risk of complex diseases are differently distributed among human populations. It is debated whether self-identified race ought to be used by medical practitioners as a proxy for the probability that an individual possesses risk-related variants.[8][9] Such practice may result in false attribution of causality, stigmatisation of high-risk populations, or underestimation of risk for other populations.[10][11] There is also a large body of evidence that environmental risk factors for complex diseases track racial categories in the United States.[5]

Genetic variation

Genetic variation arises from mutations, from natural selection, migration between populations (gene flow) and from the reshuffling of genes through sexual reproduction.[12] Mutations lead to a change in the DNA structure, as the order of the bases are rearranged. Resultantly, different polypeptide proteins are coded. . Some mutations may be positive and can help the individual survive more effectively in their environment. Variation is counteracted by natural selection and by genetic drift; note too the founder effect, when a small number of initial founders establish a population which hence starts with a correspondingly small degree of genetic variation. Epigenetic inheritance involves heritable changes in phenotype (appearance) or gene expression caused by mechanisms other than changes in the DNA sequence.

Human phenotypes are highly polygenic (dependent on interaction by many genes) and are influenced by environment as well as by genetics.

Nucleotide diversity is based on single mutations, single nucleotide polymorphisms (SNPs). The nucleotide diversity between humans is about 0.1 percent (one difference per one thousand nucleotides between two humans chosen at random). This amounts to approximately three million SNPs (since the human genome has about three billion nucleotides). There are an estimated ten million SNPs in the human population.

Research has shown that non-SNP (structural) variation accounts for more human genetic variation than single nucleotide diversity. Structural variation includes copy-number variation and results from deletions, inversions, insertions and duplications. It is estimated that approximately 0.4 percent of the genomes of unrelated people differ, apart from copy number. When copy-number variation is included, human-to-human genetic variation is estimated to be at least 0.5 percent.