Human genetic variation

Human genetic variation is the genetic differences in and among populations. There may be multiple variants of any given gene in the human population (alleles), a situation called polymorphism.

No two humans are genetically identical. Even monozygotic twins (who develop from one zygote) have infrequent genetic differences due to mutations occurring during development and gene copy-number variation.^[1] Differences between individuals, even closely related individuals, are the key to techniques such as genetic fingerprinting.

The human genome has a total length of approximately 3.2 billion base pairs (bp) across 46 chromosomes of DNA as well as slightly under 17,000 bp DNA in cellular mitochondria. In 2015, the typical difference between an individual's genome and the reference genome was estimated at 20 million base pairs (or 0.6% of the total).^[2] As of 2017, there were a total of 324 million known variants from sequenced human genomes.^[3]

Comparatively speaking, humans are a genetically homogeneous species. Although a small number of genetic variants are found more frequently in certain geographic regions or in people with ancestry from those regions, this variation accounts for a small portion (~15%) of human genome variability. The majority of variation exists within the members of each human population. For comparison, rhesus macaques exhibit 2.5-fold greater DNA sequence diversity compared to humans.^[4] These rates differ depending on what macromolecules are being analyzed. Chimpanzees have more genetic variance than humans when examining nuclear DNA, but humans have more genetic variance when examining at the level of proteins.^[5]

The lack of discontinuities in genetic distances between human populations, absence of discrete branches in the human species, and striking homogeneity of human beings globally, imply that there is no scientific basis for inferring races or subspecies in humans, and for most traits, there is much more variation within populations than between them.^{[6][7][8][9][10][11][12][13]} Despite this, modern genetic studies have found substantial average genetic differences across human populations in traits such as skin colour, bodily dimensions, lactose and starch digestion, high altitude adaptions, drug response, taste receptors, and predisposition to developing particular diseases.^[14][12] The greatest diversity is found within and among populations in Africa,^[15] and gradually declines with increasing distance from the African continent, consistent with the Out of Africa theory of human origins.^[15]

The study of human genetic variation has evolutionary significance and medical applications. It can help scientists reconstruct and understand patterns of past human migration. In medicine, study of human genetic variation may be important because some disease-causing alleles occur more often in certain population groups. For instance, the mutation for sickle-cell anemia is more often found in people with ancestry from certain sub-Saharan African, south European, Arabian, and Indian populations, due to the evolutionary pressure from mosquitos carrying malaria in these regions.

New findings show that each human has on average 60 new mutations compared to their parents.^[16][17]