Question

The EDAR gene provides instructions for making a protein called the ectodysplasin A receptor. This protein is part of a signaling pathway that plays an important role in development before birth. Specifically, it is critical for interactions between two embryonic cell layers called the ectoderm and the mesoderm. In the early embryo, these cell layers form the basis for many of the body's organs and tissues. Ectoderm-mesoderm interactions are essential for the formation of several structures that arise from the ectoderm, including the skin, hair, nails, teeth, and sweat glands. Identify two specific effects of particular EDAR alleles and the populations within which these phenotypic effects are most commonly observed.

Answer 1

Gene linked to breastfeeding may have boosted survival of earliest Americans

When the ancestors of Native Americans ventured across the Bering land bridge from today’s Siberia to Alaska about 20,000 years ago, they struggled to get enough sunlight during the long, dark winters. Living so far north with scant sunshine should have led to rickets and other health problems, yet somehow the population survived and even thrived enough to live there for thousands of years. Their lucky break, according to a new study, was that they carried a genetic mutation—revealed in ancient teeth—that boosted the development of milk ducts in women’s breasts, which may have helped nursing mothers pass more nutrients to their infants.

The gene in question is known as EDAR. Native Americans and Asians carry a version of the gene that is linked to thicker hair shafts, more sweat glands, and shovel-shaped incisors. A variant of this gene—V370A—arose about 30,000 years ago or so in China when the climate was hotter and more humid, which prompted researchers to speculate initially that it was advantageous to have more sweat glands in that environment. But the gene variant swept through the ancestors of Asians and Native Americans about 20,000 years ago, when the climate where they lived in Asia and Beringia (the now-submerged land between Asia and Alaska) was colder and dryer. So the actual cause of the gene’s spread has been unknown.

The new study reveals that the variant was so beneficial it spread to everyone in the Americas. When researchers led by biological anthropologist Leslea Hlusko of the University of California, Berkeley, examined data on the teeth of more than 5000 people from 54 archaeological sites in Europe, Asia, and North and South America, they found shoveled teeth—and hence, the gene variant that causes them—in about 40% of the individuals in Asia and all of the 3183 fossils of Native Americans they examined (who all lived before European colonization).

This suggests that some members of the first group to arrive in Beringia probably carried the gene, which arose in Asia. Then it quickly swept through the rest of the small isolated population of people who settled there between 28,000 and 18,000 years ago.

Living at such a high latitude puts nursing infants at risk of not getting enough sunlight in winter to synthesize vitamin D in their skin. Unlike adults, nursing infants can’t eat marine foods and organ meat rich in vitamin D to compensate. Vitamin D deficiency can trigger serious problems with bone development, such as rickets. It also interferes with the many ways fat insulates and fuels our bodies, as well as how the immune system wards off disease.

The EDAR variant led to the development of more elaborate branching of milk ducts in studies of mice. Hlusko and her colleagues hypothesize that those extra branches cause mothers to produce more milk or deliver more nutrients in their milk. If so, children of mothers with the EDAR variant would have been more likely to survive, thus spreading the variant throughout the population, the team proposes today in the Proceedings of the National Academy of Sciences.

The study shows how natural selection can work rapidly when humans move into extreme environments, such as the Arctic, exerting strong selection on genes critical for development and metabolism, says biological anthropologist William Leonard of Northwestern University in Evanston, Illinois, who was not involved with this work.

Follow-up studies need to be done to explore just how an increase in breast ducts might deliver more nutrients. One possibility, Hlusko says, is that the EDAR gene variant works with fatty acid genes to deliver more fats in breast milk. Researchers need to see whether there is a connection between the genes, says geneticist Tábita Hünemeier of the University of São Paulo in Brazil, who found selection for fatty acid genes in Native Americans and is not involved with the work.

Hlukso is now collaborating with others to explore how the EDAR gene variant affects breast development and density. “This is not just a Native American story,” Hlusko says. “Everyone with shovel-shaped incisors has this gene that may compensate for vitamin D deficiency.”

A Thick Head of Hair Is in the Genes

SAN DIEGO, CALIFORNIA--Whether you have a poker-straight black mane or frizzy blond locks, your hair depends on your ancestry. Now researchers have come closer to figuring out just how hair traits are passed down. According to a talk presented here last week at the annual meeting of the American Society of Human Genetics, a single genetic variant may explain why East Asians have thicker hair fibers than other populations. The discovery is part of a wave of newly found genes that determine what individuals look like.

Anthropologists have long noticed that populations across the world differ in hair type, skin color, and facial features. Finding the genes that explain these differences became easier in 2005, when researchers unveiled the International HapMap Project, a catalog of human genetic variation that helps scientists home in on genes more quickly (ScienceNOW, 26 October 2005). Geneticists at the University of Tokyo and several other institutions in Japan, Thailand, and Indonesia have now used the HapMap to explore why Japanese and Chinese people have thick hair: The cross-sectional area of East Asian hair fibers averages about 30% larger than that of Africans and 50% larger than that of Europeans.

The researchers started with 170 candidate genes known to be involved in hair development based on mouse studies and rare inherited human diseases. They then narrowed this list by looking at how these genes varied in three HapMap populations--Yoruba from Nigeria, Europeans, and Japanese and Chinese. A variant in a gene called EDAR was present in 88% of the Japanese and Chinese but not at all in the two other groups. To see how this gene affected hair thickness, the researchers examined hair fiber dimensions and DNA from 186 individuals from two ethnic groups in Southeast Asia who have a variety of hair types. Hair fibers were thickest in people who have two copies of the East Asian version of ED