Family eye genetics

Family eye genetics

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Both my parents have brown eyes. My older and younger siblings have brown eyes. I have green eyes. Is this genetically possible? Back in 1962, this was questionable and led to family problems, which have long since been resolved.

Of course it is possible. Your parents obviously have two different allels for the eye color (heterozygous), one recessive for green/blue eyes and one dominant, for brown eyes, which is expressed in this case beacuse dominant allel determines eye color if present. You and your siblings have inherited one allel from each parent. Your siblings have inherited at least one dominant (brown) allel from one of the parents, so they have brown eyes as well. You have inherited recessive allels for green eyes from both of them (homozygous). The chance to inherit two recessive allels in this case were 25%. I hope this helped you a bit :)

Eye Color Genetics

As the saying goes, &ldquothe eyes are the windows to the soul&rdquo. Our eyes are also a window into our genes. Like many of our physical features, eye color is determined by genetics.

Although there are only a few eye colors, there are many gene combinations. Examining eye color genetics can be a great way to understand genetics as a whole. In this experiment we will study eye color genetics by predicting eye color inheritance among family members.


How do we inherit our eye color?



For this experiment you can also use your own family.

  1. Observe eye color for each family member. If possible, find out the grandparents&rsquo eye colors as well. You can define the color using the Martin-Schultz scale.

  1. Determine the genotypes of the parents.
  2. Using a Punnett square, determine the color possibilities for the children.

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Symptoms Symptoms

  • mild to moderate intellectual disability
  • a distinctive facial appearance
  • and a unique personality that combines over-friendliness and high levels of empathy with anxiety.

Facial features common in young children with Williams syndrome include a broad forehead a short nose with a broad tip full cheeks and a wide mouth with full lips. In older children and adults, the face appears longer and more gaunt. Dental problems are common and may include small, widely spaced teeth and teeth that are crooked or missing. [1]

People with Williams syndrome often have outgoing, engaging personalities and tend to take an extreme interest in other people. Attention deficit disorder (ADD), problems with anxiety, and phobias are common. [1]

The most significant medical problem associated with Williams syndrome is a form of heart disease called supravalvular aortic stenosis (SVAS). SVAS is a narrowing of the large blood vessel that carries blood from the heart to the rest of the body (the aorta). If this condition is not treated, it can lead to shortness of breath, chest pain, and heart failure. The presence of other heart and blood vessel problems has also been reported. [1]

Additional signs and symptoms of Williams syndrome may include: [1]

  • abnormalities of connective tissue (tissue that supports the body's joints and organs ) such as joint problems and soft, loose skin
  • increased calcium levels in the blood (hypercalcemia) in infancy
  • developmental delays
  • problems with coordination
  • short stature
  • vision and eye problems
  • digestive problems and
  • urinary problems.

This table lists symptoms that people with this disease may have. For most diseases, symptoms will vary from person to person. People with the same disease may not have all the symptoms listed. This information comes from a database called the Human Phenotype Ontology (HPO) . The HPO collects information on symptoms that have been described in medical resources. The HPO is updated regularly. Use the HPO ID to access more in-depth information about a symptom.


Different types of albinism can have different patterns of inheritance, depending on the genetic cause of the condition. Oculocutaneous albinism (OCA) involves the eyes, hair and skin. Ocular albinism (OA), which is much less common, involves primarily the eyes, while skin and hair may appear similar or slightly lighter than that of other family members. [3] Mutations in several different genes , on different chromosomes , can cause different types of albinism.

OCA is inherited in an autosomal recessive manner. This means that two mutations are necessary for an individual to have OCA. Individuals normally have two copies of each numbered chromosome and the genes on them – one inherited from the father, the other inherited from the mother. Neither of these gene copies is functional in people with albinism. Each unaffected parent of an individual with an autosomal recessive condition carries one functional copy of the causative gene and one nonfunctional copy. They are referred to as carriers , and do not typically show signs or symptoms of the condition. Both parents must carry a defective OCA gene to have a child with albinism. [3] When two individuals who are carriers for the same autosomal recessive condition have children, with each pregnancy there is a 25% (1 in 4) risk for the child to have the condition, a 50% (1 in 2) risk for the child to be an unaffected carrier like each of the parents, and a 25% chance for the child to not have the condition and not be a carrier.

Ocular albinism type 1 is inherited in an X-linked pattern. A condition is considered X-linked if the mutated gene that causes the disorder is located on the X chromosome , one of the two sex chromosomes . In males (who have only one X chromosome and one Y), one altered copy of the causative gene in each cell is sufficient to cause the characteristic features of ocular albinism, because males do not have another X chromosome with a working copy of the gene. Because females have two copies of the X chromosome, women with only one copy of a mutation in each cell usually do not experience vision loss or other significant eye abnormalities. They may have mild changes in retinal pigmentation that can be detected during an eye examination. [4]

Researchers have also identified several other genes in which mutations can result in albinism with other features. One group of these includes at least nine genes (on different chromosomes) leading to Hermansky-Pudlak Syndrome (HPS). In addition to albinism, HPS is associated with bleeding problems and bruising. Some forms are also associated with lung and bowel disease. [3] Like OCA, HPS is inherited in an autosomal recessive manner.

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What causes heterochromia?

- A curious adult from Texas

There are several ways people can have two different colored eyes. People can be born with heterochromia, or it can result from disease or injury. It’s pretty rare for people to inherit mismatched eyes from their parents.

But before diving into the details, we need to understand where eye color comes from.

Your eyes are made up of a lot of different parts. The “whites” of the eyes are called the sclera. The dark center is called the pupil. And the colored part in between is the iris. But what gives the iris its color?

The color comes from a pigment called melanin. This is a brown pigment that causes dark hair, skin, and eyes!

The more melanin that is present in the eye’s irises, the darker the eyes will be. Brown eyes have lots of melanin and blue eyes have very little melanin.

The iris surrounds the pupil.
Image from Wikimedia.

In the case of heterochromia, each iris has a different amount of melanin. But how can this happen?

Melanin is made is special cells in the body, called melanocytes. If something goes wrong with the melanocytes in one eye, they might not make as much pigment. This can lead to two entirely different colored eyes!

But what can go wrong with these pigment cells? How can someone end up with one eye that can make pigment, while the other eye can’t?

Born with two different eyes

Heterochromia rarely runs in families. Instead, it is often caused by slight damage to the eyes. If this happens during pregnancy or just after birth, someone can be born with mismatched eyes!

But how does this happen? Why is eye color so sensitive to mild injuries or minor diseases?

It all comes back to those pigment-producing cells. It turns out these melanocytes aren’t made in the eyes. Instead, they have to travel there.

If the pigment cells get damaged, they might not make it to the eyes at all! Or maybe they’ll make it to one eye, but not the other. If one eye ends up with less melanocytes, it won’t be able to make as much pigment and will look lighter.

Other times, heterochromia at birth is caused by a larger condition or syndrome.

There are several different disorders that can cause heterochromia, including Waardenburg syndrome, Sturge-Weber syndrome, Horner's syndrome, or Parry-Romberg syndrome. All of these are rare and have other symptoms in addition to heterochromia.

In the case of Waardenburg syndrome, mutations in certain genes can result in pigmentation defects. These mutations prevent the developing melanocytes from going to the correct place. If the melanocytes don’t complete their journey the eyes, they won’t get the correct amount of pigment. This can result in either heterochromia or two very pale, blue eyes.

Waardenburg syndrome can be inherited. If one parent has it, there is a high chance the child will have it as well. And you’d probably know if you have Waardenburg, since it doesn’t just affect eye color. Most affected people have varying degrees of hearing loss as well.

And finally, there is one other (rare) way a baby can be born with heterochromia. And this one isn’t caused by injury, disease, or mutation! Two different color eyes can sometimes happen in chimeras.

Chimeras are people made up of two sets of cells, with two different sets of DNA. This can happen if the person is made up of DNA from two fused siblings. A chimera might have different eye color genes in each eye! One eye would be “your” eye color, and the other would be your “sibling’s”. (Read more about chimeras here.)

It’s rare for people to have heterochromia written in their DNA. Aside from the genetic syndromes mentioned above, heterochromia is usually caused by random chance.

But this is not true for all animals. Huskies, Australian Shepherds and Border Collies often inherit heterochromia from their parents. For dogs, having mismatched eyes is genetic!

An Australian Shepherd mix with heterochromia. Unlike in humans, many dogs have genetic heterochromia.
Photo credit: Kristen Wells

Heterochromia can be acquired later in life

Changes in eye color can also occur after birth. This usually is a result of injury or disease.

People with glaucoma sometimes end up with mismatched eyes. This disease is often treated by eye drops that can stimulate the production of melanin in the iris. This extra pigment can cause your eyes to get darker!

Tumors can also change your eye color. They can appear as light or dark patches, giving the appearance of two different colors.

Eye injury or trauma can also damage your melanocytes. If the melanocytes die, they’ll stop making pigment and your eyes will get lighter.

But heterochromia isn’t the only way that two eyes can appear to be different colors. Take the late rock-star David Bowie, for example, who had anisocoria.

Bowie got into a fight where one of his eyes got scratched. This permanently damaged the muscles that cause the pupil to contract, leaving it permanently dilated. That eye looked darker than the other, even though both were blue!

Having two different sized pupils is called anisocoria.
Image from Wikimedia

These are just a few of the ways people can end up with mismatched eyes. Eye surgery, swelling of the eye, and even diabetes have all been linked to heterochromia.

So in general, heterochromia usually occurs because something went wrong with the pigment-producing cells in our eyes. While it can be genetic, it is most often caused by injury or disease.

By Harmony Folse, Stanford University

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Insights into genetics, human biology and disease gleaned from family based genomic studies

20,000 annotated genes in the human genome are lacking. Technical advances to assess rare variation genome-wide, particularly exome sequencing (ES), enabled establishment in the United States of the National Institutes of Health (NIH)-supported Centers for Mendelian Genomics (CMGs) and have facilitated collaborative studies resulting in novel "disease gene" discoveries. Pedigree-based genomic studies and rare variant analyses in families with suspected Mendelian conditions have led to the elucidation of hundreds of novel disease genes and highlighted the impact of de novo mutational events, somatic variation underlying nononcologic traits, incompletely penetrant alleles, phenotypes with high locus heterogeneity, and multilocus pathogenic variation. Herein, we highlight CMG collaborative discoveries that have contributed to understanding both rare and common diseases and discuss opportunities for future discovery in single-locus Mendelian disorder genomics. Phenotypic annotation of all human genes development of bioinformatic tools and analytic methods exploration of non-Mendelian modes of inheritance including reduced penetrance, multilocus variation, and oligogenic inheritance construction of allelic series at a locus enhanced data sharing worldwide and integration with clinical genomics are explored. Realizing the full contribution of rare disease research to functional annotation of the human genome, and further illuminating human biology and health, will lay the foundation for the Precision Medicine Initiative.

Keywords: Centers for Mendelian Genomics (CMG) Mendelian conditions disease traits genetic models for disease rare variant phenotypes.

Conflict of interest statement

Potential Conflicts of Interest

Baylor College of Medicine (BCM) and Miraca Holdings Inc. have formed a joint venture with shared ownership and governance of Baylor Genetics (BG), formerly the Baylor Miraca Genetics Laboratories (BMGL), which performs clinical exome sequencing and Chromosomal Microarray Analysis for genome-wide detection of CNV. JRL serves on the Scientific Advisory Board of BG. JRL has stock ownership in 23andMe, is a paid consultant for Regeneron Pharmaceuticals, and is a co-inventor on multiple United States and European patents related to molecular diagnostics for inherited neuropathies, eye diseases and bacterial genomic fingerprinting. Other authors have no disclosures relevant to the manuscript.


Figure 1. CMG disease gene discovery through…

Figure 1. CMG disease gene discovery through May 30, 2018 (Year 7, Quarter 2) by…


The genetics of blood type is a relatively simple case of one locus Mendelian genetics—albeit with three alleles segregating instead of the usual two (Genetics of ABO Blood Types).

Eye color is more complicated because there's more than one locus that contributes to the color of your eyes. In this posting I'll describe the basic genetics of eye color based on two different loci. This is a standard explanation of eye color but, as we'll see later on, it doesn't explain the whole story. Let's just think of it as a convenient way to introduce the concept of independent segregation at two loci. Variation in eye color is only significant in people of European descent.

At one locus (site=gene) there are two different alleles segregating: the B allele confers brown eye color and the recessive b allele gives rise to blue eye color. At the other locus (gene) there are also two alleles: G for green or hazel eyes and g for lighter colored eyes.

The B allele will always make brown eyes regardless of what allele is present at the other locus. In other words, B is dominant over G. In order to have true blue eyes your genotype must be bbgg. If you are homozygous for the B alleles, your eyes will be darker than if you are heterozygous and if you are homozygous for the G allele, in the absence of B, then your eyes will be darker (more hazel) that if you have one one G allele.

Here's the Punnett Square matrix for a cross between two parents who are heterozygous at both alleles. This covers all the possibilities. In two-factor crosses we need to distinguish between the alleles at each locus so I've inserted a backslash (/) between the two genes to make the distinction clear. The alleles at each locus are on separate chromosomes so they segregate independently. *

As with the ABO blood groups, the possibilities along the left-hand side and at the top represent the genotypes of sperm and eggs. Each of these gamete cells will carry a single copy of the Bb alleles on one chromosome and a single copy of the Gg alleles on another chromosome.

Since there are four possible genotypes at each locus, there are sixteen possible combinations of alleles at the two loci combined. All possibilities are equally probable. The tricky part is determining the phenotype (eye color) for each of the possibilities.

According to the standard explanation, the BBGG genotype will usually result in very dark brown eyes and the bbgg genotype will usually result in very blue-gray eyes. See the examples in the eye chart at the lower-right and upper-left respectively. The combination bbGG will give rise to very green/hazel eyes. The exact color can vary so that sometimes bbGG individuals may have brown eyes and sometimes their eyes may look quite blue. (Again, this is according to the simple two-factor model.)

The relationship between genotype and phenotype is called penetrance. If the genotype always predicts the exact phenotpye then the penetrance is high. In the case of eye color we see incomplete penetrance because eye color can vary considerably for a given genotype. There are two main causes of incomplete penetrance genetic and environmental. Both of them are playing a role in eye color. There are other genes that influence the phenotype and the final color also depends on the environment. (Eye color can change during your lifetime.)

One of the most puzzling aspects of eye color genetics is accounting for the birth of brown-eyed children to blue-eyed parents. This is a real phenomenon and not just a case of mistaken fatherhood. Based on the simple two-factor model, we can guess that the parents in this case are probably bbGg with a shift toward the lighter side of a light hazel eye color. The child is bbGG where the presence of two G alleles will confer a brown eye color under some circumstances.

Genetic Variation

Genetic variation, the genetic difference between individuals, is what contributes to a species’ adaptation to its environment. In humans, genetic variation begins with an egg, about 100 million sperm, and fertilization. Fertile women ovulate roughly once per month, releasing an egg from follicles in the ovary. During the egg’s journey from the ovary through the fallopian tubes, to the uterus, a sperm may fertilize an egg.

The egg and the sperm each contain 23 chromosomes. Chromosomes are long strings of genetic material known as deoxyribonucleic acid (DNA). DNA is a helix-shaped molecule made up of nucleotide base pairs. In each chromosome, sequences of DNA make up genes that control or partially control a number of visible characteristics, known as traits, such as eye color, hair color, and so on. A single gene may have multiple possible variations, or alleles. An allele is a specific version of a gene. So, a given gene may code for the trait of hair color, and the different alleles of that gene affect which hair color an individual has. The sickle-cell allele is one version of the hemoglobin gene, and this version of the gene has a different DNA sequence from the normal version of the hemoglobin.

When a sperm and egg fuse, their 23 chromosomes pair up and create a zygote with 23 pairs of chromosomes. Therefore, each parent contributes half the genetic information carried by the offspring the resulting physical characteristics of the offspring (called the phenotype) are determined by the interaction of genetic material supplied by the parents (called the genotype). A person’s genotype is the genetic makeup of that individual. Phenotype, on the other hand, refers to the individual’s inherited physical characteristics, which are a combination of genetic and environmental influences (Figure 3).

Figure 3. (a) Genotype refers to the genetic makeup of an individual based on the genetic material (DNA) inherited from one’s parents. (b) Phenotype describes an individual’s observable characteristics, such as hair color, skin color, height, and build. (credit a: modification of work by Caroline Davis credit b: modification of work by Cory Zanker)

Most traits are controlled by multiple genes, but some traits are controlled by one gene. A characteristic like cleft chin , for example, is influenced by a single gene from each parent. In this example, we will call the gene for cleft chin “B,” and the gene for smooth chin “b.” Cleft chin is a dominant trait, which means that having the dominant allele either from one parent (Bb) or both parents (BB) will always result in the phenotype associated with the dominant allele. When someone has two copies of the same allele, they are said to be homozygous for that allele. When someone has a combination of alleles for a given gene, they are said to be heterozygous. For example, smooth chin is a recessive trait, which means that an individual will only display the smooth chin phenotype if they are homozygous for that recessive allele (bb).

Imagine that a woman with a cleft chin mates with a man with a smooth chin. What type of chin will their child have? The answer to that depends on which alleles each parent carries. If the woman is homozygous for cleft chin (BB), her offspring will always have cleft chin. It gets a little more complicated, however, if the mother is heterozygous for this gene (Bb). Since the father has a smooth chin—therefore homozygous for the recessive allele (bb)—we can expect the offspring to have a 50% chance of having a cleft chin and a 50% chance of having a smooth chin (Figure 4).

Figure 4. (a) A Punnett square is a tool used to predict how genes will interact in the production of offspring. The capital B represents the dominant allele, and the lowercase b represents the recessive allele. In the example of the cleft chin, where B is cleft chin (dominant allele), wherever a pair contains the dominant allele, B, you can expect a cleft chin phenotype. You can expect a smooth chin phenotype only when there are two copies of the recessive allele, bb. (b) A cleft chin, shown here, is an inherited trait.

Sickle-cell anemia is just one of many genetic disorders caused by the pairing of two recessive genes. For example, phenylketonuria (PKU) is a condition in which individuals lack an enzyme that normally converts harmful amino acids into harmless byproducts. If someone with this condition goes untreated, he or she will experience significant deficits in cognitive function, seizures, and increased risk of various psychiatric disorders. Because PKU is a recessive trait, each parent must have at least one copy of the recessive allele in order to produce a child with the condition (Figure 5).

So far, we have discussed traits that involve just one gene, but few human characteristics are controlled by a single gene. Most traits are polygenic: controlled by more than one gene. Height is one example of a polygenic trait, as are skin color and weight.

Figure 5. In this Punnett square, N represents the normal allele, and p represents the recessive allele that is associated with PKU. If two individuals mate who are both heterozygous for the allele associated with PKU, their offspring have a 25% chance of expressing the PKU phenotype.

Where do harmful genes that contribute to diseases like PKU come from? Gene mutations provide one source of harmful genes. A mutation is a sudden, permanent change in a gene. While many mutations can be harmful or lethal, once in a while, a mutation benefits an individual by giving that person an advantage over those who do not have the mutation. Recall that the theory of evolution asserts that individuals best adapted to their particular environments are more likely to reproduce and pass on their genes to future generations. In order for this process to occur, there must be competition—more technically, there must be variability in genes (and resultant traits) that allow for variation in adaptability to the environment. If a population consisted of identical individuals, then any dramatic changes in the environment would affect everyone in the same way, and there would be no variation in selection. In contrast, diversity in genes and associated traits allows some individuals to perform slightly better than others when faced with environmental change. This creates a distinct advantage for individuals best suited for their environments in terms of successful reproduction and genetic transmission.

Epigenetic Research, Healing, and the Global Village

The more we understand about ourselves, the traumas we’ve inherited, and the hurt we may harbor, the greater our responsibility to harness this awareness for change.

The more we understand about ourselves, the traumas we’ve inherited, and the hurt we may harbor, the greater our responsibility to harness this awareness for change.

Watch the video: Οικογενειακές Ιστορίες 1292017 Μπροστά στα μάτια μου (July 2022).


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