What Color Will My Baby's Eyes Be?

How Genetics Determine Eye Color

Baby Eye Color

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Eye color is determined by genetics. The color of your eyes, whether blue, brown, hazel, or green, is ultimately the result of multiple genes and polymorphisms (variations of a gene). Although parents-to-be often wonder what color their baby's eyes will be, there is really no way to make a 100% accurate prediction.

Most of the genes associated with eye color regulate the production of a brown to yellowish-brown pigment called melanin (the same pigment involved in hair and skin color). The amount and quality of melanin in the front layers of the iris determine what the ultimate color will be.

Beyond aesthetics, eye color may be predictive of ocular (eye) diseases, particularly if there is a change eye color. Color patterns in the iris are now even being used for personal identification just like fingerprints, with iris scans replacing pin codes in many high-security settings.

Anatomy of Eye Color

The iris is a thin, circular structure inside the eye responsible for changing the diameter and size of the pupil. This controls how much light enters the eyes and reaches the retina. In addition to acting as the eye's aperture, the iris has a thin layer of cells called the iris pigment epithelium which disperses melanin into the stroma (the fibrous and vascular layer) of the iris.

The amount of melanin excreted by the cells and absorbed by the stroma will determine whether the iris is brown, hazel, green, gray, blue, or variation of those colors. The perception of color is based not only to how much melanin is absorbed but the pattern of disbursement in the stroma.

The absence of melanin, for example, will be perceived as blue or gray as the pigment is only modestly scattered and selectively absorbed by stromal structures called melanosomes. By contrast, melanosomes that absorbed more melanin will be perceived in darker shades of brown. The complete absence of melanin accounts for the pale eye color seen in some people with albinism.

Because melanin is yet not amply distributed in newborns, they will often have blue eyes when born. This will often change in the weeks or months that follow as the pigment epithelial cells start to pump melanin into the stroma.

Eye Color Variations

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

There may also be variations in eye color 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. The contrasting, variegated tones are often clearly seen up-close. However, 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 their presence is of no clinical significance.

In the end, eye color is determined not by pigment alone but by the structure and distribution in melanosomes.

Genetics of Eye Color

Eye color is determined largely by your 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 that eye color was determined by a single gene, with brown eyes being dominant over blue. With the advent of genetic research and genomic mapping, scientists today understand 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 a 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—there is just as strong a chance that blue-eyed parents could end up with a brown-eyed baby.

With that said, patterns do exist that may help predict the likelihood of eye color. One traditional 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 that 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 with 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 are also uncommon. Roughly 5% to 8% of people will have hazel-colored eyes. Amber eyes (caused when pheomelanin dominates the iris) is extremely rare, manifesting 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 can often 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 is 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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