"Four: Some scientists have revealed that most whites are unable to produce melanin because their pineal glands are often calcified or non-functioning. Pineal calcification rates with Africans are five to 15 percent, Asians 15 to 25 percent and Europeans 60 to 80 percent. This is the chemical basis for the cultural differences between Blacks and whites.
The MC1R gene provides instructions for making a protein called the melanocortin 1 receptor, located on the surface of melanocytes that produce melanin. MC1R evolved in response to the selective pressure of ultra-violet radiation (UVR). Melanocytes make two forms of melanin, eumelanin and pheomelanin. Selection to maintain darker skin in high UVR environments occurred in African populations, producing mostly eumelanin. On the other hand, European people in low UVR environments produce mostly pheomelanin and have light-colored skin, which is further enhanced by SLC24A5 and SLC45A2, two golden genes related to the evolution of the light skin color.
Evolutionary model of human pigmentation in three continental populations. The rooted tree shows the genetic phylogeny of human populations from Africa, North Europe and East Asia, with the colors of the branches roughly indicating the generalized skin pigmetation level of these populations (adapted from McEvoy et al. (2006) ). Genetic loci reported to be under positive selection in the common ancestor of modern Eurasians are represented by rs1881227 in KITLG, and those independently evolved in Europeans and East Asians, indicating possible convergent evolution, are represented by rs12913832 in OCA2 and rs885479 in MC1R, respectively. The maps of allele frequency were drawn using R (version 3.2.1, https://www.r-project.org
), based on these loci in 53 global populations provided by the Human Genome Diversity Panel CEPH (HGDP, http://www.hagsc.org/hgdp/index.html
). Blue and red colors denote the ancestral and derived alleles, respectively
Skin color adaptation in modern Eurasians
In Europeans, SLC24A5 and SLC45A2 [16,17,18,19] are two golden genes related to the evolution of the light skin color. SLC24A5 encodes the NCKX5 protein, which is a member of the transmembrane protein family and regulates the calcium concentration in the melanosome . This gene has been confirmed to affect pigmentation in zebrafish and mice [16, 20]. Especially, the derived allele of rs1426654 in SLC24A5 was found to be nearly fixed in Europeans, but almost missing in populations without any European ancestry (Fig. 2) . A 78-kb haplotype around SLC24A5, which is in linkage disequilibrium with rs1426654, was also identified to accumulate in Europeans . A similar pattern can be observed at rs16891982 in SLC45A2 , which has been reported to be associated with pigmentation in several species, e.g., mice, fish, birds, and horses [24,25,26]. Other variants in this gene, including rs26722, rs2287949, and rs40132, were also shown to be coloration-associated in Europeans [23, 27, 28]. Another important pigmentation-related gene identified in European is MC1R [29,30,31]. This gene is expressed in melanocytes and plays a key role in controlling the switch from pheomelanin to eumelanin . The pigmentary phenotypes associated with MC1R has been studied in a wide range of animals [32,33,34]. Many variants have been identified in MC1R, such as rs1805007, rs1805008, and rs3212357 [35, 36], despite its small size (951 bp). Other important European-specific loci include rs1393350 in TYR, rs2733831 in TYRP1, and rs1900758 in OCA2 [17, 28, 37,38,39]. The derived allele frequencies at these loci are high in Europeans but low in Africans and East Asians, which could be a clear signal of positive selection in Europeans, as indicated by statistical analysis .
Genes involved in the skin color adaptation in East Asians are not that well studied compared to the long list of adaptive genes identified in Europeans. Notable examples include OCA2 and MC1R. Each harbors several non-synonymous mutations, e.g. rs1800414 and rs74653330 in OCA2, and rs885479 in MC1R [40,41,42,43], which exhibit high derived allele frequencies in East Asians, but low derived allele frequencies in Europeans and Africans (Fig. 2). The OCA2 protein is thought to be a mature melanosomal membrane protein , with a potential role in protein transportation into melanosomes . The East Asian-specific variant of rs1800414 was first reported in an exome sequencing study aiming to figure out albinism-related variants . The derived allele at rs1800414 was thought to contribute to the skin lightening in an association study of Han Chinese, which measured the skin color of individuals using the melanin index . Another non-synonymous variant in OCA2, rs74653330, has also been confirmed to be pigmentation-related in an association study of Japanese . Additional examples of East Asian-specific pigmentation-associated alleles include rs10809814 in TYRP1 and rs1407995 in DCT [40, 49], both of which show differentiation between Asians and non-Asians , and strong signals of positive selections in Asians [43, 49].
https://hereditasjournal.biomedcentral. ... 017-0036-2
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