Eye colour often is the genetic trait that fascinates parents the most as a child develops. Will the child’s eyes be black, brown, blue, gray, green, hazel or some combination of colours?
How a child looks depends on the genetic material each parent contributes to the child. But the parents’ genes can mix and match in many different ways. The influences from each parent aren’t known until after the child is born.
How eye colour develops
The colored part of the eye, known as the iris, contains pigments that determine eye color.
Human eye color is primarily determined by three genes, two of which are well understood. These genes account for the most common eye colors—green, brown, and blue. However, the genetic basis for other colors like gray, hazel, and various combinations remains less clear.
It was once believed that brown eye color was “dominant” and blue was “recessive.” However, modern genetics has revealed that eye color inheritance is more complex.
Eye color is not a simple blend of the parents’ colors, like mixing paint. Each parent carries two pairs of genes on each chromosome, and there are multiple ways this genetic information can influence eye color.
In early childhood, eye color can change.
Many non-Hispanic Caucasian infants are born with blue eyes, which may darken during the first three years of life. This darkening occurs if melanin, a brown pigment typically absent at birth, develops over time.
Children may inherit eye colors that differ from either of their parents. However, if both parents have brown eyes, their children are more likely to also have brown eyes.
Darker eye colors tend to be dominant, so brown often prevails over green, and green usually over blue.
Yet, if one parent has brown eyes and the other has blue, it doesn’t guarantee a brown-eyed child.
Some children are born with irises of different colors, a condition known as heterochromia. This can result from abnormal pigment development, trauma before or after birth, or a benign genetic condition. Other possible causes include inflammation, an iris freckle (diffuse nevus), or Horner’s syndrome.
If you notice any unusual changes in your eye color, it’s important to consult an eye doctor.
Changes in Eye Colour
The iris is a muscle that controls the size of the pupil, which expands in low light and contracts in bright light. The pupil also becomes smaller when focusing on close objects, like reading a book.
As the pupil size changes, the pigments in the iris either compress or spread out, causing a slight change in eye color.
Emotions can also affect both pupil size and iris color. This is why some people notice their eyes appear to change color when they’re feeling strong emotions like anger or love.
Eye color can also change with age. This occurs in about 10 to 15 percent of Caucasians, who generally have lighter eye colors.
For example, eyes that were once a deep brown might lighten to hazel, a mix of brown and green. However, some hazel eyes may actually darken as one ages.
If your eye color changes significantly in adulthood, or if one eye changes from brown to green or from blue to brown, it’s important to consult an eye doctor.
Such changes can indicate underlying health conditions, such as Fuch’s heterochromic iridocyclitis, Horner’s syndrome, or pigmentary glaucoma.