In the array of possible natural hair colors, dark hues are the most common — more than 90 percent of people worldwide have brown or black hair. That’s followed by blonde hair.

Red hair, occurring in just 1 to 2 percent of the population, is the least common. Blue eyes are similarly uncommon, and they may be becoming rarer.

One study found that between 1899 and 1905, more than half of non-Hispanic white people in the United States had blue eyes. But from 1936 to 1951, that number fell to 33.8 percent. Today, estimates suggest about 17 percent of people worldwide have blue eyes.

Your hair color and eye color come down to what genes you inherit from your parents. If one person has both red hair and blue eyes, there’s a good chance one or both of their parents do, too, but not always.

You must inherit two sets of genetic information for both your hair color and your eye color to have these less-common characteristics. The likelihood of this happening is quite rare, especially if neither of your parents has red hair or blue eyes. Sometimes, however, the genetic stars align, and individuals are born with the rare combination of red hair and blue eyes.

Gene characteristics fall into two categories: recessive and dominant. Parents share the blueprint of many features, from hair color to personality, in their genes.

Though hair color is influenced by multiple genes, in general, dominant genes win out in a head-to-head matchup against recessive genes. Brown hair and brown eyes, for example, are both dominant, which is why they make up such a large percentage of hair-eye color combinations.

Parents can also be carriers for recessive genes. While they may display the dominant genes, they still have — and can pass to their kids — the recessive genes. For example, two brown-haired, brown-eyed parents can have a child with blonde hair and blue eyes.

Both parents can display recessive gene characteristics, and they can pass those to their children, too. For example, if both parents have red hair, a child receives mostly the genetic information for red hair, so the chances they will have red hair is almost 100 percent.

If one parent is redheaded and the other isn’t, the chances their child will have red hair is about 50 percent, though the shade of red may vary greatly.

Lastly, if both parents are carriers of the gene variant but don’t have red hair, the child has about a 1 in 4 chance of having truly red hair. The true pattern of inheritance of hair color is somewhat more complicated, though, as there are many genes involved.

What gene causes red hair?

Melanocytes are melanin-forming cells in your skin. The amount and type of melanin your body produces determines how dark or light your skin will be. Red hair is the result of a genetic variant that causes the body’s skin cells and hair cells to produce more of one particular type of melanin and less of another.

Most redheads have a gene mutation in the melanocortin 1 receptor (MC1R). When MC1R is inactivated, the body produces more pheomelanin, which is responsible for reddish skin and hair tones, than eumelanin, which is responsible for shades of brown and black. In people with an activated MC1R, eumelanin can balance pheomelanin, but in redheads, the gene variant prevents that.

Whether you have one or both MC1R gene copies inactivated can also determine the shade of red hair you have, from strawberry blonde to deep auburn to bright red. This gene is responsible for freckles in many redheads, too.

Are red-haired, blue-eyed people going extinct?

You might believe that because these genetic traits are rare, they could be diluted out of the gene pool entirely. That’s not likely to happen. Even when you can’t see recessive characteristics — red hair, for example — they’re still there, hiding out in a person’s chromosomes.

When a person has a child, they can pass their recessive gene information to their offspring, and the trait could win out. That’s why something like red hair or blue eyes could “skip” generations and show up a few steps down the family line.

Red hair is more common in women, according to research. However, Caucasian males are more likely to have blue eyes than females, research shows. As for the combination of the red hair and blue eyes, little research has looked at which sex is more likely to develop this uncommon characteristic combo.

Redheads know their hair color isn’t the only unique characteristic. In fact, redheads have some other rare tendencies.

Limited research suggests redheads may be more likely to be left-handed. Like red hair, left-handedness is a recessive trait. In the Western hemisphere, 10 to 15 percent of people use their left hand dominantly.

Redheads are thought to be more sensitive to pain, too, research shows. Plus, they may require more anesthetic during surgery or local anesthesia.

While redheads are born all throughout the world, they’re more likely to crop up in the Northern hemisphere. Approximately 2 to 6 percent of redheads call the upper half of the globe home.