Today’s Wonder of the Day was inspired by julia. julia Wonders, “What Determines Eye Colour?” Thanks for WONDERing with us, julia!
There are two main factors that help determine your eye color: the amount and pattern of dark brown pigment (called melanin) in the part of your eye called the iris and the way in which the iris scatters light that passes through the eye. The more important factor is pigment, which is determined by your genes.
Chromosomes are comprised of pieces of DNA called genes. These genes, which also come in pairs, determine many of your characteristics you will develop. Scientists believe that as many as 16 different genes could play a role in determining eye color. The two main genes believed to be responsible are OCA2 and HERC2, both of which are part of chromosome 15.
Genes are, in turn, made up of alleles that ultimately determine whether any particular characteristic will appear. For each trait you can inherit, there are two alleles. If the two alleles are the same, they are homozygous. If they are different, they are heterozygous. For each trait, one allele (dominant) is expressed (the trait it represents appears), while the other allele (recessive) is unexpressed (the trait it represents does not appear). Recessive alleles are only expressed if there is no dominant allele present.
For example, the alleles for eye color can be separated into blue, green, and brown. Green alleles are dominant over blue alleles, and brown alleles are dominant over both blue and green alleles. If you received a blue allele and a brown allele, your eye color would be brown because brown is the dominant allele. If you have blue eyes, that means you received blue alleles from both parents.
Your genes also determine your eye color by dictating how much (and where) melanin is produced in your iris. The more melanin produced, the darker the eye color will be. Because melanin production does not begin at birth, babies' eyes appear blue. True eye color will be determined over time. It's usually not until age three that a child's true permanent eye color reveals itself.
Have you ever noticed how some people's eyes seem to change color depending upon the lighting? That occurs because the iris has two layers. Sometimes there is pigment in both layers. In people with blue or green eyes, however, the front layer will have very little or no melanin. Depending upon the amount and diffraction of light, their eyes may appear to change colors.
Some people have two different eye colors. This results from a condition called heterochromia. It's very rare, but usually harmless. It occurs due to differences in the early stages of iris development.