Red hair is a relatively rare human characteristic, naturally occurring in only about 2% of the global population, though it is more frequent in individuals of Northern and Northwestern European ancestry. The distinct reddish hue is the result of a specific biological process controlled by genetics. Red hair is determined by an autosomal recessive pattern of inheritance. This genetic mechanism explains why the trait can appear in a family where neither parent has the phenotype.
The Gene Responsible for Red Hair
The molecular mechanism for red hair is controlled by the Melanocortin 1 Receptor (MC1R) gene, located on chromosome 16. This gene provides instructions for creating the melanocortin 1 receptor protein, which is located on the surface of pigment-producing cells called melanocytes. The receptor’s function is to regulate the type of melanin produced, the pigment that gives color to skin, hair, and eyes.
When the MC1R protein is fully functional, it signals the melanocyte to produce eumelanin, the dark, brownish-black pigment. Eumelanin is responsible for dark hair and skin that tans easily, offering protection against ultraviolet (UV) radiation. In people with red hair, the MC1R gene carries specific mutations that cause the receptor to be less functional or inactive.
This loss of function prevents eumelanin production, causing the melanocytes to produce a different pigment called pheomelanin. Pheomelanin is a reddish-yellow pigment that gives red hair its characteristic color. While MC1R is the main genetic determinant, a variety of mutations in this gene, known as Red Hair Color (RHC) variants, can lead to the phenotype.
Understanding Recessive Inheritance
Genetic traits are passed down through alleles, which are different versions of a gene inherited from each parent. Red hair is a recessive trait, meaning an individual must inherit two copies of the mutated MC1R allele—one from each parent—to express the red hair phenotype. The inheritance pattern is called autosomal recessive because the gene is located on a non-sex chromosome, an autosome.
A person with two copies of the mutated allele is described as homozygous for the red hair trait and will have red hair. Conversely, a person who inherits only one copy of the mutated MC1R allele and one copy of the functional allele will not have red hair. The functional allele is dominant and produces enough eumelanin to result in darker hair, such as brown or blonde.
This individual with one functional and one mutated allele is known as a carrier; they possess the genetic information for red hair but do not display the trait themselves. When two carriers have a child, there is approximately a 25% chance that the child will inherit a mutated allele from both parents and be born with red hair. The frequency of carriers is much higher than the frequency of redheads; for example, in Scotland, about 40% of the population carries the gene, while only 13% have red hair.
Common Traits Associated with Red Hair
The genetic change in the MC1R receptor that causes red hair also has consequences for other physical traits. The reduced production of dark eumelanin throughout the body results in very fair skin and an inability to tan effectively. This fair skin, often accompanied by freckles, confers a significantly increased sensitivity to UV radiation and a higher risk for sun damage and certain skin cancers, including melanoma.
The same MC1R gene variants have been linked to differences in pain perception and how individuals respond to certain medications. Studies suggest that redheads may require a notably higher dose of general anesthesia to remain sedated during surgery, sometimes up to 20% more than non-redheads. This difference is thought to be due to the receptor’s role in a pathway that influences the central nervous system’s response to pain.
Redheads may also be more sensitive to specific types of pain, particularly thermal pain from heat and cold, but less sensitive to certain skin-stinging pains, like those from electric shock or needle pricks. Furthermore, some research indicates that redheads may be more responsive to certain opioid pain medications. Due to these physiological differences, individuals with red hair may need to inform their medical providers about their anesthetic requirements before procedures.