Light entering the eye is focused by the lens onto a light-sensitive panel of cells known as the retina at the rear of the eye. It is here that the light is detected and converted into electrical signals by photoreceptor cells of which there are two types: cone cells are responsible for sharp vision and colour vision while rod cells (named for their cylindrical shape) are highly sensitive to light and so are responsible for night and peripheral vision. These then transmit the signals to the brain via the optic nerve.

Previous assumptions were that the inheritance of eye colour was relatively simple involving only one gene with a version (allele) for brown eyes dominant over blue eyes. However, the genetic basis for eye colour is actually far more complex and is a polygenetic trait i.e. governed by the presence of more than one gene. At least 6 genes are known to interact with each other to give the various colour shades and so almost any parent-child combination of eye colours can occur.

The most common eye colour is brown, while blue eyes are found mainly in people of Northern European and Eastern European descent. Green is one of the rarest eye colours, though is fairly common among Pashtuns (e.g. Sharbat Gula the “Afghan Girl” from the June 1985 cover of National Geographic) as well as people of Celtic, Germanic and Slavic descent.

autosomal recessive

1:10,000

Defects in cone cells result in colour blindness and hemeralopia, often known as day blindness where an extreme sensitivity to light can occur. One inherited disease characterised by this is Stargardt’s disease. Here mutations in a gene, (ABCA4), producing a protein involved in providing energy to retinal cells, leads to the gradual deterioration of cone cells and central vision, along with the ability to perceive colours, while leaving peripheral vision intact. Symptoms begin in childhood where there is wavy vision, blind spots, blurriness, impaired colour vision, and difficulty adapting to dim lighting.

The athlete Marla Runyan, though having competed in the Olympic final in Sydney 2000 for the 1,500 meters run, suffers from Stargardt’s disease. Her vision was perfectly normal until she was about 9 years old at which time she began to experience problems seeing the blackboard in school and reading books. As a result of this condition Marla Runyan is legally blind but is still able to see shadows and peripheral details. In her biography, “No Finish Line”, she recalls being amazed to discover that other hurdlers could see all ten hurdles on the track, when she could barely see the first one! It was this that led her to switch to distance running and she still races competitively.

In the 2010 Winter Olympics, the Canadian cross-country skier Brian McKeever who also suffers from Stargardt disease became the first winter athlete to compete in both the Paralympic and Olympic games.

Several actresses with sectoral heterochromia (two colours in a single eye) are Kate Bosworth who has a hazel section at the bottom of her right otherwise blue eye, Jessica Cauffiel who hasbrown pigmentation in her left eye and Elizabeth Berkley whose right eye is half green and half brown. Heterochromia is also particularly common in dogs such as Dalmatians and Border Collies. David Bowie supposedly does not have heterochromia – his discoloured eye is actually a permanently dilated pupil that he sustained after getting punched in a fight over a girl when he was 13. Marilyn Manson gives the appearance of having this trait through the use of differently coloured contact lenses.

A number of movie actors have eye colour irregularities, the presence of which may play some roles in creating a more striking appearance. For example, Dan Aykroyd has one green eye and one brown eye and Jane Seymour has one green eye and one brown eye, Kiefer Sutherland and Dan Ackroyd also have differently coloured eyes.

Heterochromia describes, the occurrence of eyes with separate colours. This is frequently genetic in origin, and often autosomal dominant inherited. In addition there are some inherited syndromes which present with this feature in a combination with other symptoms, for example Waardenburg syndrome, which is caused by mutations in a developmental gene (pax3) leading to pigment disturbances in the iris, hair and skin, as well as hearing loss. Piebaldism and Hirschsprung’s disease also sometimes present with heterochromia.

autosomal recessive, dominant, sex-linked, mitochondrial inherited

1:3,000

Retinitis pigmentosa is one of the most common forms of inherited retinal degeneration typically characterized by the progressive loss of rod cells leading to night-blindness followed later by a reduction of the peripheral visual field (known as tunnel vision). Presently, 35 different genes are known to cause this disease when mutated and consequently the disease shows many different patterns of inheritance depending upon which gene is affected.

Former child actor Isaac Lidsky who played the role of Weasel on “Saved by the Bell: The New Class” was diagnosed with the disease at age 13. Two of his sisters also inherited the same gene and also have the same condition. Giving up acting, he studied Applied Mathematics and Computer Science at Harvard College and in 2008 became the first legally blind US Supreme Court clerk.

One of Mexico’s biggest music stars, Rigo Tovar, suffered from RP. Losing his sight in his 20’s, he would constantly wear dark sunglasses to protect his retina, although many people did not understand he suffered from RP believing it to be an affectation.

1:15,000

An uncontrolled growth of the retina, leading to a cancer of this tissue, can caused by the inheritance of mutations in a gene called the retinoblastoma-1 gene. This gene produces a protein that functions in normal cells as a tumour suppressor acting as a brake on cell division. This disease, known as retinoblastoma affects either one or both eyes in around children. However, half of cases appear to be non-heritable.

Peter Falk, the American actor, best known for his role as Lieutenant Columbo, suffered from retinoblastoma as a child losing his right eye at the age of three and consequently wears a glass eye.

The English comedienne, actress and writer Caroline Ahern is partially sighted in one eye as a result of suffering this in childhood, as well as her brother who also inherited the same gene.

Myopia (short-sighted vision) is the most common eye problem in the world. Affecting a quarter of the population, it occurs when the lens of the eye is unable to flatten properly resulting in light entering the eye to focus in front of the retina. Myopia is hereditary to a large degree, though one direct gene has yet to be discovered and so it is though a number of genes may be interacting to cause the defect. However, severe mutations in a gene known as pax6 that is particularly influential in eye development can lead to a massive underdevelopment of the eye while less disruptive alterations in this same gene appear to associate with cases of myopia with the eye adopting a slightly elongated front to back shape which affects focusing.

The visual defect usually presents during the pre-teen years, worsens gradually as the eye grows during adolescence, and then levels off as a person reaches adulthood.

It has been suggested that the field of the Impressionism may owe its existence to the poor eyesight associated with myopia. Artists such as Monet, Renoir and Degas supposedly suffered myopia or short-sightedness which may have influenced their painting style, with several of the rejecting the use of spectacles.

Glaucoma (Gr. glaukos; blue-grey – possibly first used to describe cataracts) is a term used for a group of diseases that can lead to damage to the eye’s optic nerve and result in blindness. This generally occurs when the fluid pressure inside the eye is too high. Affecting around 1 in 200 people it does not generally exhibit Mendelian inheritance though relatives of people who have the condition have a significantly higher risk of developing the condition themselves. There are, however, a small number of cases of familial glaucoma where mutations in a number of key genes lead to a very high susceptibility, usually at an earlier age. The most common mutation is found in the GLCA1 gene on chromosome 1 which produces a protein involved in pressure regulation of the eye, the disruption of which can lead to increased intraocular pressure. It is this increased intraocular pressure that is the biggest risk factor for glaucoma and can lead to optic nerve degeneration.

The great Argentinean writer Jorge Luis Borges also lost his sight to glaucoma, which he had inherited from his father who had become blind in middle age, as had other relations on his father’s side of the family. “In my case, that slow nightfall, that slow loss of sight, began when I began to see. It has continued since 1899 without dramatic moments, a slow nightfall that has lasted more than three-quarters of a century. In 1955 the pathetic moment came when I knew I had lost my sight, my readers and writer’s sight”.

The disease can manifest itself as acute, where sight is lost very quickly, or chronic. The latter, often known as “the silent sight thief”, leads to a gradual, often unnoticed, impairment of vision taking place over a long period of time. John Milton, considered one of the greatest poets of the English language, wrote his epic poem Paradise Lost (1667) after he became blind from glaucoma. It was while he was in his 30s, that he felt his sight getting weak and dull, until by the age of 43 he was totally blind.

Johann Sebastian Bach went blind in a violent flash, suggesting that he may have suffered from the acute form of glaucoma. He died months after a futile, and possibly harmful, operation on his eyes in 1750.

8% of males, 0.4% of females
sex-linked

The most common deficiency is red-green colour blindness, generally affecting males as the genes for two of the photoreceptors involved in this disorder are on the X chromosome. This results in a lack of perception of red, orange, green, blue and cyan.

Many people are unaware of their problem which can often only be revealed by tests such as an Ishihara diagram. Alternatively, problems could be encountered in trying to accomplish certain tasks involving distinguishing colour, such as chemical titration.

Amazingly, John Dalton, one of England’s most renowned chemists, was red-green colour blind and was actually one of the first to describe the condition. He correctly surmised that it must be hereditary, since his brother had the same visual defect. Mistakenly though, he was convinced that this defect resulted from the lens of his eye being tinted blue, and he requested that after his death (in 1844) his eyes be removed and examined to confirm this. Even though after his death his lenses were found not to be tinted, for many years colour blindness was called Daltonism, after him. However, the potential problems associated with red-green colour blindness were not widely appreciated until 1875, when a train accident in Sweden was attributed to the driver’s colour blindness.

Interestingly, while as many as 8 percent of North Europeans are affected by this disorder, there are considerably lower frequencies in other ethnic groups. One possible explanation is that individuals with colour blindness may develop better night vision. It is suggested, therefore, that the prevalence of colour deficiencies may associate with the duration of twilight; longer twilight periods, at higher latitudes, may have resulted in improved hunting skills for colour blind individuals in these regions. The famous Canadian World War I combat pilot and aviation daredevil, Erroll Boyd, was turned down by the Royal Flying Corps because of his colour blindness. However, he was accepted by the Royal Naval Air Service, where he excelled as a night pilot. He also became the first person to fly across the North Atlantic, in 1930, outside the summer season. Later on, Boyd recalled: “Boy, it was dark! I felt as though I was piloting a car in a coal mine.”

The greyish-green paintings of John Constable have led many to suggest that the great landscape artist might have harboured a colour deficiency, along with other notable artists such as James Whistler, Charles Meryon, Fernand Leger and Piet Mondrian, all of whom also characteristically used more restricted pallets. Some artists, to compensate for their colour deficiency, have resorted to using the brightest colours possible – interestingly, there are a number of comic-book artists who have described having a red-green colour deficiency.

Autosomal recessive

1:33,000

Total colour blindness, known as achromatopsia, are a group of rare inherited conditions where individuals can only differentiate shades of gray. This can result from an inability of cone cells to transmit signals and react to light.

Many of the inhabitants of the small coral island of Pingelap, in the Western Pacific Ocean, are unable to distinguish any colours. A freak storm in the eighteenth century killed most of the islanders leaving only around twenty women and a handful of men including a young chief who happened to be a carrier of the gene for achromatopsia. Consequently, around a third of the atoll’s 3,000 inhabitants now carry the gene leading to 6 percent of the population suffering from the disorder.

The Canadian singer, songwriter frontman of Steppenwolf, John Kay, also has total colourblindness.

mitochondrially inherited

1:30,000

Degeneration of the optic nerve that transmits visual information from the retina to the brain, leads to irreversible blindness in young adulthood. There are two types of inherited optic atrophy both of which effect the functioning of mitochondria, and so possibly disrupting energy utilization in the nerve tissues, in particular the optic nerve. One disease, known as Kjer’s optic neuropathy, is inherited dominantly through any of several genes on various chromosomes which control the stability of mitochondria. The other, Leber’s hereditary optic neuropathy, is a mitochondrial inherited disease due to a mutation of the mitochondrial genome and hence is passed exclusively through the mothers.

A large number of French-Canadians sufferers in Quebec, suffer from Leber’s hereditary optic neuropathy where it is often referred to as the Frenchman Disease. This serves as another example of a founder effect. When Canada became a royal province in 1663, there was an unfavourable ratio of six male colonists to every European-born female. With a view to reducing this imbalance and to ensuring the stability of the colony, the French King Louis XIV devised a program whereby over 700 young single French women were sent out between 1663 and 1673. Most of these women were orphans who had already been placed under the care of the king and so became known as the “filles du roy,” or “King’s daughters”. Many of the 5 million French-Canadians living in Quebec province are descendants of these women. One, however, carried a single nucleotide change in her mitochondrial DNA. Marrying one of the colonists in Quebec City in 1669, she produced five daughters who also all married in or near Quebec City. Today, as many as 90 percent of French-Canadians affected with Leber’s hereditary optic neuropathy can trace their ancestry back to this woman.

Leber’s congenital amaurosis is an inherited defect in both rod and cone cells and so characterised by blindness at birth. There are as many as 10 different causal genes leading to the abnormal development of both types of photoreceptor cells.

Scott MacIntyre, the American singer, songwriter, and pianist, as well as the eighth place finalist on the eighth season of American Idol has only a two-percent field of vision and suffers from tunnel vision as a result of this form of blindness.

Singaporean singer, Kelvin Tan Wei Lian, who was also born with this was a street busker before winning a national competion and secruing a recording contract with Warner music

This disease affected a now very famous dog by the name of Lancelot. Totally blind at birth due to the inheritance of a gene for this disease, he was injected with a genetically engineered virus into his eye which carried a healthy copy of the gene to his retina. As a result, Lancelot can now catch a ball and has given hope for the possibility of sight restoration through gene-therapy.

Albinism, resulting from defects in melanin production also leads to visual defects, with individuals generally showing sensitivity to light due to the lack of any melanin in the iris absorbing any stray light entering the eye. However, the major vision defects in people with albinism result from an abnormality in an area of the retina important for sharp vision such as reading. This tissue requires melanin to properly develop nerve connections between the retina and the brain. When these connections are disrupted involuntary movements of the eyes, known as nystagmus, can occur in addition to strabismus where the eyes are unable to fixate together, i.e. ’cross-eyed’.

Dr. Spooner, the famous Oxford classicist, may have had this condition. It is possible that nystagmus, associated with his albinism, caused a jumbling of information from the printed page which led to his famous speech aberration. Strabismus, affecting around 5 percent of people, can lead to a loss of binocular vision and depth perception. A number of the world’s most famous artists may have actually owed their talents, to some degree, to this condition.

Rembrandt’s divergent squint, evident in some of his self portraits, has led to the suggestion that this contributed to his ability to translate a 3-dimentional world to a 2-dimentional canvas.

Albrecht Durer, who has been credited by some have credited with formalising and refining linear perspective in Renaissance Art, is another artist who suffered congenital strabismus, as did the great French artist Edgar Degas who once commented to a friend: “One sees what one wants to see and the falsehood constitutes art”.