It is a common belief that a visually distinct trait like red hair must be dominant, requiring only one parent to pass it on. This is a frequent misconception about human genetics. Red hair, which ranges from bright copper to deep auburn, is governed by a precise genetic mechanism involving pigment production. Understanding this mechanism clarifies how the hair color appears across generations.
Red Hair is a Recessive Trait
Red hair is classified as an autosomal recessive trait. The gene variant responsible must be inherited from both biological parents for the trait to be fully expressed. Non-red hair gene versions are generally dominant, masking the red hair trait when only one copy is present. The genetic change associated with red hair is located on chromosome 16.
An individual who inherits one variant copy and one non-red hair copy will not have red hair, but they are known as a carrier. A carrier possesses the genetic information and can pass it on, even though the trait is not physically visible. Up to 40% of people in some populations, like Ireland, may carry this recessive gene.
How the $MC1R$ Gene Creates Red Pigment
The specific gene responsible for red hair is the Melanocortin 1 Receptor, or $MC1R$. This gene provides instructions for making the $MC1R$ receptor protein, which is found on the surface of pigment-producing cells called melanocytes. The receptor’s primary function is to control the type of melanin pigment melanocytes produce.
Melanocytes produce two main forms of melanin: eumelanin (a dark brown or black pigment) and pheomelanin (a lighter, reddish-yellow pigment). When the $MC1R$ receptor functions normally, a hormone activates it, triggering the production of dark eumelanin. If a person inherits two variant copies of the $MC1R$ gene, the receptor’s function is reduced or blocked. This prevents the switch to eumelanin production, causing melanocytes to produce mostly reddish pheomelanin, resulting in red hair.
Calculating Inheritance Probability
Understanding the recessive nature of red hair allows for calculating inheritance likelihoods.
Carrier Parents
When two non-red-haired parents are both carriers of the $MC1R$ variant, there is a 25% chance their child will inherit two copies and have red hair. There is a 50% chance the child will be a non-red-haired carrier, and a 25% chance the child will not be a carrier.
Red-Haired and Carrier Parents
If one parent has red hair (two variant copies) and the other is a non-red-haired carrier, the probability changes significantly. There is a 50% chance the child will inherit red hair and a 50% chance the child will be a non-red-haired carrier.
Two Red-Haired Parents
When both parents have red hair, their child will almost always inherit red hair because each parent can only contribute the variant gene.
Other Genetic Features Linked to Red Hair
The genetic change in the $MC1R$ gene influences other physical characteristics due to its role in overall pigmentation. The reduced ability to produce dark eumelanin results in fair skin highly sensitive to ultraviolet (UV) radiation. This fair skin, which contains higher levels of pheomelanin, tans poorly and is associated with an increased tendency to develop freckles.
The $MC1R$ gene mutation is also linked to an altered perception of pain and temperature. Studies suggest that redheads may respond differently to certain types of pain relief and may be more sensitive to temperature changes. While $MC1R$ is the main factor, researchers have identified other genetic differences that work by controlling when $MC1R$ is switched on or off, suggesting hair color inheritance involves a network of genes.