Genetics of eye color

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Expecting parents often wonder what their baby will be like. One of the most frequently asked questions is what color your child's eyes will be. But although eye color is determined by genetics , it takes a child a year to develop a consistent eye color.

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Parents may wonder why their bulging blue-eyed child flaunts brown eyes as a child. This is because the baby's eye color will change during the first year of life as the eye takes on a consistent color.

Understanding how the eyes acquire their color and the role that genetics plays can reveal part of the mystery behind this phenomenon. Although eye color is primarily a physical characteristic, in some cases it can be a sign that a child has a health problem.

Scientists once believed that eye color is determined by a gene, but advances in genetic research and genomic mapping have shown that more than a dozen genes affect eye color.

In this article, we'll take a look at how genetics decides what color your child's eyes are.

How eye color develops

The colored part of the eye is called the iris . What we see under eye color is actually just a combination of pigments (colors) that form in the layer of the iris known as the stroma. There are three of those pigments:

  • Melanin is a yellow-brown pigment that also determines skin tone.
  • Theomelanin is Orange-red pigment responsible for red hair. It is seen most often in people with green and brown eyes.
  • Eumelanin is a black-brown pigment found abundantly in dark eyes. Determine how intense the color will be.

The combination of pigments, as well as their distribution and absorption by the stroma, determines whether the eye appears brown, hazel, green, gray, blue, or a variation of these colors.

For example, brown eyes contain more melanin than green or hazel eyes. There is very little pigment in blue eyes. They appear blue for the same reason that the sky and water appear blue, due to light scattering, thus reflecting more blue light.

When you don't have any melanin, you get the pale blue eyes of people with albinism .

A newborn's eyes are usually dark, and the color often depends on their skin tone. White babies are usually born with blue or gray eyes. Black, Latino, and Asian babies often have brown or black eyes.

When a baby is born, the pigment does not spread through the iris. During the first six months of life, more pigments are produced. By the age of 1, he usually has permanent eye color.

Genetics and eye color

Eye color is determined by many variations in the genes responsible for the production and distribution of melanin, pheomelanin, and eumelanin. The main genes that influence eye color are called OCA2 and HERC2. Both are found on human chromosome 15.

Each gene has two different versions (alleles). You inherit one from your mother and one from your father. If the two alleles of a particular gene are different ( heterozygous ), the dominant trait is expressed (displayed). The hidden trait is called recessive.

If a trait is recessive, such as blue eyes, it usually only appears when the alleles are the same ( homozygous ).

Brown eye color is a dominant trait and blue eye color is a recessive trait. Green eye color is a combination of both. Green is recessive to brown, but prevails over blue.

Predict eye color

Without knowing exactly what genes a child will have, it is impossible to predict with complete certainty what color their eyes will be. But there are ways to make fairly accurate predictions.

One is to use a simple grid graph called a Pennett square. You enter the genetic characteristics of one of the parents at the top of the table. The genetic traits of the other parent are recorded in the leftmost columns. The graph of each parent's contribution gives a greater than average probability of their child's eye color.

Identifying the alleles for each parent can be a bit tricky depending on the color of the eyes. Brown eyes can be the dominant trait of six different genetic combinations. They can also hide recessive (hidden) green or blue eye traits. Knowing your grandparents' eye color is helpful in finding recessive signs.

For example, a blue-eyed father whose entire family has blue eyes and a brown-eyed father whose mother and father were both brown-eyed and blue-eyed have a 50/50 chance of having a blue-eyed or blue-eyed child. brown. .

Eye color probability
Parent 1 Father 2 Blue Green Brown
Blue Blue 99% 1% 0%
Blue Green fifty% fifty% 0%
Blue Brown fifty% 0% fifty%
Green Green 25% 75% 0%
Green Brown 12% 38% fifty%
Brown Brown 19% 7% 75%

Scientists have begun to develop methods to predict eye color. They use genetic tests that identify specific polymorphisms (versions of genes) that can indicate the amount of melanin, pheomelanin, and eumelanin that will be produced.

Eye color and health

A baby's eye color can also indicate congenital diseases (diseases he was born with) and other conditions. Babies with different colored eyes, known as heterochromia, can have Waardenburg syndrome .

It is a genetic disorder that can cause hearing loss in one or both ears. People with Waardenburg syndrome can also be born with very light or two-colored eyes.

Very pale blue eyes may be due to ocular albinism . This is when there is absolutely no pigment in the iris.

Ocular albinism as an X-linked recessive disease occurs almost exclusively in men. This is because men have an X sex chromosome and a Y chromosome. The gene for this disease is located on the X chromosome. Thus, in men, the gene for this disease will be expressed, even if it is recessive.

On the other hand, women have two X sex chromosomes, so they can be carriers. They may have a gene for ocular albinism that is hidden by another normal gene. Therefore, they may not have the disease themselves, but they can pass on the gene that causes it.

Studies show that less than one in 60,000 men suffers from ocular albinism .

A baby can also be born without all or part of the iris, a genetic disorder known as aniridia . It is caused by mutations in the PAX6 gene. This gene plays an important role in the formation of tissues and organs during the development of the embryo.

Summary

Your child's eye color is determined by genetics. Eye color is a combination of pigments produced by the stroma. Brown eyes have more melanin than green or brown eyes. There is very little pigment in blue eyes.

The combination of genes inherited from each parent determines which pigments are produced and the baby's eye color. These genes can also cause certain conditions.

Get the word of drug information

While understanding the genetics of eye color can help you understand the likelihood of a baby having a particular eye color, there is no certainty. If you have any questions about your child's eye color or general eye health, please contact your pediatrician.

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