Science: Map: How Human Skin colour works… Why we have White Skins

[All our distinguishing traits come down to our evolutionary history. Below is mention of why we have a lighter skin. But I will be returning to the topic of White Intelligence. Jan]

Human skin color is a “polygenic” trait, meaning multiple gene loci are involved in its expression. At last count, the International Federation of Pigment Cell Society has determined that there are a total of 378 genetic loci involved in determining skin color in human and mice. Among them, only 171 have been cloned and though the other 207 loci have been mapped out, the true gene identities have yet to be determined.

One subgroup of the cloned genes is related to melanosomes and their precursors. Melanosomes are the granules within melanocytes where melanin is synthesized. In humans, melanin is the primary determinant of skin color and is also found in hair and the iris. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and its reduced forms. Most eumelanins are derived from the amino acid tyrosine. The following diagram summarizes the biogenesis pathway of melanin in the human melanocyte and its transportation to adjacent keratinocytes. Not surprisingly, many genes depicted in this figure are the so-called “color genes.”

For example, the gene MC1R is located at the cellular membrane of the melanocyte. It is also known as a melanocyte-stimulating hormone receptor (MSHR), melanin-activating peptide receptor, or melanotropin receptor. When activated by α-MSH, MC1R initiates a complex signaling cascade that switches on the TYRP1 gene with the help of MITF, the master regulator of melanocyte lineage. TYRP1 and TYP (tyrosinase) are both needed to augment the production of the brown or black pigment eumelanin. Mutations, or SNPs (single nucleotide polymorphism) of MC1R, such as Arg151Sys (rs1805007), Arg160Trp (rs1805008), Asp294Sys (rs1805009), Val60Leu (rs1805005) and Val92Met (rs2228479), have been shown to cause red hair and pale skin in a small percentage of the human population.

Another gene called SLC24A5 (Solute carrier family 24 member 5) regulates calcium in melanocytes but its exact role in melanogenesis is still unknown. Evidence suggests that the Thr111Ala allele (SNP type rs1426654) is a major factor in the light skin tone of Europeans. All Asian and African populations carry the original allele, while the mutant allele is found in about 99.9% of Europeans. A single base pair substitution is believed to represent around 25 to 40% of the difference in skin tone between Europeans and Africans.

Another gene called SLC24A5 (Solute carrier family 24 member 5) regulates calcium in melanocytes but its exact role in melanogenesis is still unknown. Evidence suggests that the Thr111Ala allele (SNP type rs1426654) is a major factor in the light skin tone of Europeans. All Asian and African populations carry the original allele, while the mutant allele is found in about 99.9% of Europeans. A single base pair substitution is believed to represent around 25 to 40% of the difference in skin tone between Europeans and Africans.

The dominance of the mutant SLC24A5 allele in the European population suggests a strong selective pressure for paler skin tones in regions with low levels of UV radiation possibly involving the regulation of Vitamin D synthesis, frost bite sensitivity and cold tolerance.

Calcitrol, the biologically-active form of vitamin D, circulates as a hormone and regulates the concentration of calcium and phosphate in the bloodstream while promoting healthy bone growth. The prohormone, or previtamin D, is synthesized in the skin by UV-B from the sun. UV-B exposure causes a photochemical (non-enzymatic) conversion of 7-DHC (or 7-dehydrocholesterol ) into previtamin D. High melanin content in skin reduces UV-B exposure and reduces photochemical conversion making pale skin more advantageous in areas with low sun exposure. However, UV-A rays from sunlight can also cause folic acid deficiency in light skinned people. Lack of folic acid causes anemia and problems in cell division and growth, which is especially important in infancy and pregnancy. Extended exposure to ultraviolet light also causes damage to the DNA of skin cells, which can lead to skin cancer and other complications. Based on this, it has been theorized that Asian and African populations kept the original form of SLC24A5 and benefited from darker skin that helped neutralize heavy sun exposure. A glance at the map of indigenous skin color distribution around the world based on Von Luschan’s chromatic scale seems to support that theory.

When the mutant allele of SLC24A5 was first described in Science in 2005, it was incorrectly dubbed the “white gene.” As mentioned earlier, skin color is a polygenic trait and depends on the expression of multiple genes. People with the original SLC24A5 allele (the “black allele,” as it’s sometimes referred to) can still develop albinism, as can many of the aforementioned “color genes.”

Although the genetic basis for skin color is complicated as its social implications, it is clear that genetic variations that contribute to skin color also affect our heath. When interviewed by Scientific American magazine in 2005, Dr. Shriver, the co-discoverer of SLC24A5 mutant allele, said:
“We know so little about the genetic and evolutionary architecture of human traits. We cannot expect to use human genetics to understand complex diseases most effectively without first working out how fundamental characteristics, such as eye, hair and skin color, are determined.”

Source: https://www.gbhealthwatch.com/Trait-Skin-Color.php