Areas that receive higher amounts of UVR, generally located closer to the equator, tend to have darker-skinned populations. Areas that are far from the tropics and closer to the poles have lower intensity of UVR, which is reflected in lighter-skinned populations. Some researchers suggest that human populations over the past 50,000 years have changed from dark-skinned to light-skinned and vice versa as they migrated to different UV zones, and that such major changes in pigmentation may have happened in as little as 100 generations (≈2,500 years) through selective sweeps. Natural skin color can also darken as a result of tanning due to exposure to sunlight. The leading theory is that skin color adapts to intense sunlight irradiation to provide partial protection against the ultraviolet fraction that produces damage and thus mutations in the DNA of the skin cells. In addition, it has been observed that females on average are significantly lighter in skin pigmentation than males. Females need more calcium during pregnancy and lactation. The body synthesizes vitamin D from sunlight, which helps it absorb calcium. Females evolved to have lighter skin so their bodies absorb more calcium.The social significance of differences in skin color has varied across cultures and over time, as demonstrated with regard to social status and discrimination. Melanin and genes
Melanin is produced by cells called melanocytes in a process called melanogenesis. Melanin is made within small membrane–bound packages called melanosomes. As they become full of melanin, they move into the slender arms of melanocytes, from where they are transferred to the keratinocytes.
Dietary copper can be found in whole grain cereals, legumes, oysters, organ meats (particularly liver), cherries, dark chocolate, fruits, leafy green vegetables, nuts, poultry, prunes, and soybean products like tofu.Copper deficiency can have many hematological consequences, such as myelodysplasia, anemia, low white blood cell count, and low count of neutrophils (a type of white blood cell that is often called "the first line of defense" of the immune system). Copper deficiency has long been known as a cause of myelodysplasia (when a blood profile has indicators of possible future leukemia development), but it was not until 2001 that copper deficiency was associated with neurological manifestations like sensory ataxia (irregular coordination due to proprioceptive loss), spasticity, muscle weakness, and more rarely visual loss due to damage in the peripheral nerves, myelopathy (disease of the spinal cord), and rarely optic neuropathy. Signs and symptoms
Blood symptoms
The characteristic hematological (blood) effects of copper deficiency are anemia (which may be microcytic, normocytic or macrocytic) and neutropenia. Thrombocytopenia (low blood platelets) is unusual.The peripheral blood and bone marrow aspirate findings in copper deficiency can mimic myelodysplastic syndrome. Bone marrow aspirate in both conditions may show dysplasia of blood cell precursors and the presence of ring sideroblasts (erythroblasts containing multiple iron granules around the nucleus).