Question

Include all the information below about Skin color and sun exposure : ( its genetic mutation(s), and the evolutionary mechanisms via which your trait came to differ between populations)

What is the trait called colloquially and scientifically?

What are the hallmarks or characteristics? ?

What mutation(s) causes the trait in your population of interest?

Name of allele? Gene? Type of mutation? ?

What are the differences in frequencies of your trait between the two populations, or, which alleles are characteristic of each population? ?

Are there aspects of this trait that could alter the fitness of the population?

Was the trait selected for or against? What about the environment caused this selection? ?

Did migration, bottlenecking, or the founder effect play pivotal roles in the prevalence of your trait? ?

Were there any types of selective breeding that contributed to the difference in frequency between populations. ?

Is there evidence that interactions with other, pre-existing genes, played a role in the emergence or extinction of your trait within each population?

Answer 1

The most influential division of skin colour is that into five categories due to Johann Friedrich Blumenbach (1779), which divides mankind into "Caucasian or White", "Mongolian or Yellow", "Aethiopian or Black", "American or Copper-colored" and "Malayan or Olive-colored" subgroups. However, it has been found that exposure to sunlight have profound effect on skin color.

Human skin color ranges in variety from the darkest brown to the lightest hues. An individual's skin pigmentation is the result of genetics, being the product of both of the individual's biological parents' genetic makeup. In evolution, skin pigmentation in human beings evolved by a process of natural selection primarily to regulate the amount of ultraviolet radiation penetrating the skin, controlling its biochemical effects

Human skin color is a “polygenic” trait, meaning multiple gene loci are involved in its expression. In polygenic inheritance, the genes contributing to a trait have equal influence and the alleles for the gene have an additive effect. Polygenic traits do not exhibit complete dominance as do Mendelian traits, but exhibit incomplete dominance. In incomplete dominance, one allele does not completely dominate or mask another. The phenotype is a mixture of the phenotypes inherited from the parent alleles. Environmental factors can also influence polygenic traits. 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.

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. On exposure to sunlight the melanin spreads throughout the body and the skin becomes dark. When melanin is not spread throughout the body, the person appears fair. Spreading of melanin is also due to protect the body from harmful UV rays of sunlight.

Polygenic traits tend to have a bell-shaped distribution in a population. Most individuals inherit various combinations of dominant and recessive alleles. These individuals fall in the middle range of the curve, which represents the average range for a particular trait.

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.

Migration of people to different regions of the world where temperature varies has also resulted in variation of skin color. People who are on or near the tropics are darker than those who stay near the poles. In the subsequent generations, this change has been inherited. Migration has changed the gene frequencies of the earlier generations.