The Genetics of Red Hair

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Red hair is caused by recessive mutations at the melanocortin 1 receptor gene.
[BestPhotoStudio/Shutterstock.]

Whether because of its exotic hue or its novelty, red hair has long been a subject of fascination for historians, poets, artists, and scientists. Historians made special note of the fact that Boudica, the Celtic queen who led a revolt against the Roman Empire, possessed a “great mass of red hair.” Early Christian artists frequently portrayed Mary Magdalene as a striking redhead (though there is no mention of her red hair in the Bible), and the famous artist Botticelli painted the goddess Venus as a red-haired beauty in his masterpiece The Birth of Venus. Queen Elizabeth I of England possessed curly red hair; during her reign, red hair was quite fashionable in London society.

The color of our hair is caused largely by a pigment called melanin that comes in two primary forms: eumelanin, which is black or brown, and pheomelanin, which is red or yellow. The color of a person’s hair is determined by two factors: (1) the amount of melanin produced (more melanin causes darker hair; less melanin causes lighter hair) and (2) the relative amounts of eumelanin and pheomelanin (more eumelanin produces black or brown hair; more pheomelanin produces red or blond hair). The color of our hair is not just an academic curiosity; melanin protects against the harmful effects of sunlight, and people with red hair are usually fair skinned and particularly susceptible to skin cancer.

The inheritance of red hair has long been a subject of scientific debate. In 1909, Charles and Gertrude Davenport speculated on the inheritance of hair color in humans. Charles Davenport was an early enthusiast of genetics, particularly of inheritance in humans, and was the first director of the Biological Laboratory in Cold Spring Harbor, New York. He later became a leading proponent of eugenics, a movement—now discredited—that advocated improvement of the human race through genetics. The Davenports’ study was based on family histories sent in by untrained amateurs and was methodologically flawed, but their results suggested that red hair is recessive to black and brown, meaning that a person must inherit two copies of a red-hair gene—one from each parent—to have red hair. Subsequent research contradicted this initial conclusion, suggesting that red hair is inherited instead as a dominant trait and that a person will have red hair even if possessing only a single red-hair gene. Controversy over whether red hair color is dominant or recessive or even dependent on combinations of several different genes continued for many years.

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In 1993, scientists who were investigating a gene that affects the color of fur in mice discovered that the gene encodes the melanocortin-1 receptor. This receptor, when activated, increases the production of black eumelanin and decreases the production of red pheomelanin, resulting in black or brown fur. Shortly thereafter, the same melanocortin-1 receptor gene (MC1R) was located on human chromosome 16 and analyzed. When this gene is mutated in humans, red hair results. Most people with red hair carry two defective copies of the MC1R gene, which means that the trait is recessive (as originally proposed by the Davenports back in 1909). However, from 10% to 20% of redheads possess only a single mutant copy of MC1R, muddling the recessive interpretation of red hair (the people with a single mutant copy of the gene tend to have lighter red hair than those who harbor two mutant copies). The type and frequency of mutations at the MC1R gene vary widely among human populations, accounting for ethnic differences in the preponderance of red hair: among those of African and Asian descent, mutations for red hair are uncommon, whereas almost 40% of the people in the northern part of the United Kingdom carry at least one mutant copy of the gene for red hair.

Modern humans are not the only people with red hair. Analysis of DNA from ancient bones indicates that some Neanderthals also carried a mutation in the MC1R gene that almost certainly caused red hair, but the mutation is distinct from those seen in modern humans.

This chapter is about the principles of heredity: how genes—such as the one for the melanocortin-1 receptor—are passed from generation to generation and how factors such as dominance influence that inheritance. The principles of heredity were first put forth by Gregor Mendel, and so we begin this chapter by examining Mendel’s scientific achievements. We then turn to simple genetic crosses, those in which a single characteristic is examined. We will consider some techniques for predicting the outcome of genetic crosses and then turn to crosses in which two or more characteristics are examined. We will see how the principles applied to simple genetic crosses and the ratios of offspring that they produce serve as the key for understanding more -complicated crosses. The chapter ends with a discussion of statistical tests for analyzing crosses.

Throughout this chapter, a number of concepts are interwoven: Mendel’s principles of segregation and independent assortment, probability, and the behavior of chromosomes. These concepts might at first appear to be unrelated, but they are actually different views of the same phenomenon because the genes that undergo segregation and independent assortment are located on chromosomes. This chapter aims to examine these different views and to clarify their relations.