What Color Will My Baby's Eyes Be?

How Genetics Determine Eye Color

Baby Eye Color

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Eye color, whether blue, brown, hazel, or green, is determined by genetics, the result of multiple genes and polymorphisms (variations of a gene). Most genes associated with eye color regulate the production of a brown to yellowish-brown pigment called melanin (the same pigment that directs hair and skin color). The amount and quality of melanin in the front layers of the iris determine eye color.

The significance of eye color extends beyond esthetics. Color patterns in the iris are now being used for personal identification just like fingerprints, with iris scans replacing pin codes in high-security settings.And changes in eye color can be predictive of ocular diseases.

Anatomy of Eye Color

The iris of the eye which is responsible for changing the diameter and size of the pupil to control how much light enters the eyes, has a thin layer of cells called the iris pigment epithelium. These cells disperse melanin into the stroma (the fibrous and vascular layer) of the iris, where it is selectively absorbed by structures called melanosomes.The amount of melanin excreted by the cells and absorbed by the stroma determine whether the iris is brown, hazel, green, gray, blue, or a variation.

The perception of color is based not only on how much melanin is absorbed but the pattern of disbursement in the stroma.When melanin is only modestly scattered eye color is perceived to be blue or gray. When a larger and denser amount of melanin is absorbed, eyes are brown. The complete absence of melanin accounts for the pale eye color seen in some people with albinism.

Because melanin is not amply distributed in newborns, they often have blue eyes at birth. This can change after several weeks or months as the pigment epithelial cells start to pump melanin into the stroma.

Eye Color Variations

Melanin is not the only pigment in the eye. There are two subtypes: pheomelanin (which is reddish-orange and also is largely responsible for red hair) and eumelanin (which is a blackish-brown and determines how dark an eye, skin, or hair color will be). Depending on how much pheomelanin and eumelanin is disbursed, eye may appear greenish, hazel, or amber.

There also may be color variations within the iris itself. Because melanin is generally distributed outward from the pupil, there may be more browns in the center of the iris and more blues and grey in the periphery. These contrasting, variegated tones often can be seen up close, although from a distance, we tend to perceive them as a solid color.

Speckles in the eye can occur when there are elevations or depressions in the stroma. This can lead to dark or light spots in the iris, neither of which are inherently problematic. With that said, whitish spots arranged in a concentric circle around the pupil are common in children with Down syndrome (trisomy 21), although they aren't clinically significant.

Genetics of Eye Color

Eye color is determined largely by genetics. Genes provide the blueprint by which proteins—the building blocks of the body—are assembled. This, in turn, determines the structure of cells and how they work. The collection of genes responsible for your genetic traits (referred to as your genotype) directs the characteristics you inherit as an individual (referred to as phenotype).

With respect to eye color, genetics will determine how much melanin, pheomelanin, and eumelanin is secreted by epithelial cells, as well as the structure, distribution, and susceptibility of melanosomes.

Scientists once believed eye color was determined by a single gene, with brown eyes being dominant over blue. With the advent of genetic research and genomic mapping, it's now known that eye color is influenced by multiple genes.

The two genes most commonly associated with eye color are the OCA2 and HERC2 genes located on human chromosome 15.

Each of these genes comes in two different versions, known as alleles, for a total of four alleles. If the two alleles are different (heterozygous), only one is expressed and is considered dominant. The other unexpressed allele is referred to as recessive. If a trait is recessive, like blue eyes, it generally only appears when two recessive alleles are the same (homozygous).

Brown eye color is dominant to green, and green eye color is dominant to blue. Therefore, blue eyes can only occur when all alleles are blue.

Besides these two important genes, there are at least 10 other genes that affect eye color, each combination of which results in a different eye phenotype.

Eye Color Probability

From the point of view of probability, it seems logical that two blue-eyed parents would only produce blue-eyed babies. However, because multiple genes and gene interactions are involved—some of which may be unexpressed in the parents—the likelihood blue-eyed parents might have a baby with brown eyes is just as likely.

With that said, patterns do exist that may help predict eye color. One tool is called a Punnett square developed by geneticist Reginal Punnett in the early 20th century. Using a grid chart, the genetic traits of one parent are entered in the top rows of the chart, while the genetic traits of the other parent are entered in the far left columns.

The genetic trait is described in letters based on a review of each parent's history (such as the color of their parent's eyes). For example, a blue-eyed parent whose family has always been blue-eyed might be assigned the letters "aa," while the brown-eyed parent whose mother and father were brown- and blue-eyed might be given an "Aa."

A single letter is placed in each of the respective grids. By plotting the contribution each parent makes, the Punnett square can provide a better-than-average probability of their child's eye color. In the above-listed example, there is a 50/50 chance that the child will either blue-eyed with two recessive genes (aa) or brown-eyed with one dominant and one recessive gene (Aa).

Given the complexity of human genetics, the Punnett square has its limitations and still leaves a lot open to chance.

In recent years, scientists have begun to develop methods for predicting eye color using genetic tests that identify specific polymorphisms. Based on which polymorphism are found, scientists can better predict how much melanin, pheomelanin, and eumelanin will be produced as well as the degree of saturation in the iris.

While it will be some time before such a tool is available and fully automated, geneticists strongly believe doctors will soon be able to predict eye color and other physical traits with a high degree of accuracy.

Eye Colors Worldwide

Brown is the most common eye color in the world: Between 55% and 79% of the people of the world having brown eyes. Blue and green eyes are genetically recessive, so they are far less common. Less than 10% of the world's population is believed to have blue eyes, while only around 2% has green eyes.

Hazel and amber eyes also are uncommon. Roughly 5% to 8% of people will have hazel-colored eyes. Amber eyes (caused when pheomelanin dominates the iris) is extremely rare, appearing in less than 1% of people.

Ocular albinism, in which there is a total absence of iris pigmentation, is the rarest condition of all. As an X-linked recessive disorder, ocular albinism occurs almost exclusively in men since they have two X chromosomes. (Women, who have an X and a Y chromosome, may be carriers.) Studies suggest that less than one of every 60,000 men is affected.

Eye Color and Health

The color of your eyes may help predict your risk of certain eye diseases. People with lighter eyes, for example, are more likely to develop age-related macular degeneration (ARMD) than those with darker eyes. The same applies to uveal melanoma (a type of eye cancer) in which lighter eyes and fair skin are key risk factors.

By contrast, people with dark brown eyes are believed to be at a greater risk of developing cataracts later in life.

On rare occasions, a person may have two different colored eyes, a condition referred to as heterochromia. It may occur naturally with no long-term problems or consequences. However, if it develops later in life, it is often the sign of a serious condition, such as chronic iritis (an inflammatory disorder that can lead to blindness) or diffuse iris melanoma (a type of eye cancer).

Eye trauma can also cause a sudden and significant change in eye color.

When to See a Doctor

Any change in eye color, however minor, should not be ignored. This is especially true if the change is unilateral (occurring on one side). Bilateral changes are no less concerning as this may the sign of cataracts or glaucoma.

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