The Biology of Skin Color, by HHMI Biointeractive

Which of the following statements about melanin is most accurate?

• Melanin is found on in human cells and is not present in other organisms.

• Human skin cells contain only one type of melanin.

• The more eumelanin an individual has in their skin cells, the darker their skin.

• The darker an individual's skin color, the more likely ultraviolet (UV) radiation will damage the DNA in their cells.

Based on the film, which of the following statements best describes the pigments in a red tomato?

• The pigments reflect red light and absorb blue and green light.

• The pigments absorb red light and reflect blue and green light.

• The pigments will protect DNA in the tomato's cells from damage caused by UV light.

• The tomato fruit has the same types and amounts of pigments as in the plant's leaves, stem, and roots.

Which of the following areas is likely to have the highest amount of UV radiation?

• an area at a low altitude that is usually cloudy

• an area at a high altitude near the equator

• an area near the ocean or other large body of water

• an area at a longitude in the Western Hemisphere

Which of the following statements accurately describe variation in human skin color? Select all that apply.

• Data support a correlation between UV intensity and skin color variation.

• Human skin color variation is mostly determined by the types and amounts of melanin present in the skin.

• Variations in human skin color can be quantified, or measured.

• On average, the skin color of indigenous peoples around the world correlates with the UV intensity in their environments.

Darker skin is more prevalent among indigenous people living in areas with high amounts of UV radiation. Nina Jablonski proposed a hypothesis to explain the selective pressure for darker skin in these environments. On which of the following observations did she base this hypothesis?

• Human populations around the globe have different versions of the MC1R gene.

• In general, people with more darkly pigmented skin have less folate, a nutrient important for human reproduction, circulating through their blood.

• The larger amount of eumelanin in darkly pigmented skin protects circulating folate from being destroyed by UV radiation.

• The melanin in darkly pigmented skin protects DNA from UV radiation, which can cause skin cancer.

Indigenous populations in low-UV environments tend to have lighter skin tones. One hypothesis is that, in low-UV environments, the selective pressure for dark skin is low. Instead, there is selective pressure for lighter skin, which absorbs more UV radiation, since UV is needed for vitamin D production. Which of the following statements best supports this hypothesis?

• In general, people with lighter skin tend to have a higher risk of vitamin D deficiency.

• Red hair and light skin are most common among people of the northern European descent.

• Indigenous peoples with darker skin who live at high altitudes have diets that consist of foods rich in vitamin D.

• People with lighter skin cannot synthesize enough vitamin D in high-UV environments, such as equatorial Africa.

Rickets, a disease that affects bone development in children, is caused by vitamin D deficiency. Based on the risk factors discussed in the film, which of the following groups of people has the highest risk of rickets if they do not take vitamin D supplements? Assume that each group spends a lot of time outdoors.

• adults with dark skin who live close to the equator

• children with light skin who live close to the equator

• adults with light skin who live far south of the equator

• children with dark skin who live far north of the equator

Skin color in humans is an example of an evolutionary trade-off: a situation where a trait that is an advantage in one context is a disadvantage in a different context. Darker skin protects DNA from damage due to ultraviolet radiation (an advantage) but makes it harder for the body to produce vitamin D (a disadvantage). Based on what you’ve learned, which of the following describe other examples of evolutionary trade-offs? Select all that apply.

• Some genetic diseases that protect humans from malaria, a disease transmitted by mosquitoes, also cause the blood disease thalassemia.

• Some garter snakes have mutations that make them resistant to a toxin found in their prey, rough-skinned newts. Snakes with these mutations move more slowly and may not be as good at hunting or avoiding predators.

• Alleles that caused the loss of body hair allowed humans to lose heat more easily in hot environments. This in turn helped humans walk and run longer distances to hunt large animals.

• The common Mormon butterfly is not toxic to birds and other predators. Some common Mormons have genetic variations that make them resemble toxic butterflies, which deters predators. In places without predators, common Mormons with these variations don't live as long as other common Mormons.

If you travel north from the equator, what generally happens to the intensity of ultraviolet (UV) light?

• The intensity increases.

• The intensity decreases.

• The intensity stays the same.

• It is impossible to predict.

Who would you expect to be most at risk for developing the bone disease rickets?

• Children born to mothers with dark skin, living far from the equator

• Adults with dark skin who live close to the equator

• Children born to mothers with light skin, living close to the equator

• Adults with light skin who live close to the equator

• Anyone who eats a diet that includes a lot of fish