This week’s question was asked by a kid at school.
QUESTION: Why do all babies start out with blue eyes?
ANSWER: The pupil is that black center of the eye, and it is black for everyone. The iris of the eye is that thin circular structure responsible for controlling the diameter and size of the pupil. The iris of the eye is like a diaphragm allowing more light or less light to strike the retina where the image is formed. It is the iris of the eye that is colored.
Eye color is an inherited trait from parents. There was a once held view that blue eyes were caused by a recessive trait. Scientists now know that eye color is much more complex, involving as many as 14 different genes. In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46. The two main genes associated with eye color are located on Chromosome 15.
Pigmentation of the iris in humans varies depending on the amount of melanin, a natural brown pigment, located in the back of the iris. Most babies of European ancestry have light colored, or blue eyes, at birth. As a child develops, from age one to three, cells in the iris slowly begin to produce melanin. If sufficient amounts of melanin are produced, the eyes turn brown or green.
Babies of African American, Hispanic and Asian descent are usually born with dark eyes that stay that way. This is because these non-white ethnicities naturally have more pigment in their skin, hair, and eyes. Melanin is more abundant in darker individuals.
Eye color is also dependent on the scattering of light in the stroma, a delicate interlacing of fibers that circle the circumference of the iris and radiate toward the pupil. This fine mesh is interspersed with blood vessels and nerves. This dispersing of light, referred to as Rayleigh scattering, is the same phenomena that accounts for the blueness of the sky.
Lighting and angle can have a large impact on the appearance of eye color. The color of clothing and makeup can also make the color of the eyes appear different, though the actual color remains the same. Sunlight can make eyes appear lighter, and dilated pupils can make eyes appear darker. Light-colored eyes reflect more light and are more likely to appear different colors.
Hazel is a mixture of green and brown, sometimes with yellow gold thrown in. Hazel is tricky, because hazel-colored eyes appear to change color, appearing more medium golden-dark green and sometimes a light brown. Hazel eyes are due to a moderate amount of melanin in the iris border layer and that Rayleigh scattering.
Hazel is oft referred to as “mood eyes” because the color depends of the colors a person is wearing and the lighting. Hazel eyes have flecks and ripples of green, gold and red, so a color change for hazel-eyed people is quite common.
It turns out that most eye-colored changes occur in Caucasian populations with hazel and amber eyes. There is strong evidence from twins, both fraternal and identical, that the changes are genetically determined.
A question that frequently arises: Is there a relationship between eye color and personality? Probably not a scientific study, but brown eyes have been associated with intelligence, blue eyes with sweetness and kindness, green eyes with being creative. People with hazel eyes encompass all the above characteristics of intelligence, sweetness, kindness and creativity. Of course, that description comes from hazel-eyed people!
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