Bones Variation
Accessory ossicles
Small extra bones, most often in the foot, that form when a growth centre never fuses with its main bone. They are harmless but can be mistaken for fractures on an X-ray.
Up to 1 in 5 feet
Genetic
Has a known or suspected genetic basis.
35 oddities.
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Brain Trick
Ambidexterity
Using both hands equally well for every task. Fewer than 1% of people are truly ambidextrous. Most people who say they are ambidextrous are mixed-handed: right hand for some jobs, left for others.
Under 1%
Ears Variation
Attached and free earlobes
Some earlobes hang free and others join the head directly. Textbooks taught this as one gene, but a 2017 study found at least 49 genetic regions involved, and the two shapes blend into each other.
Both shapes are common
Mouth Variation
Bifid uvula
The uvula, the dangling flap at the back of the throat, is split into two lobes, in about 1 to 2% of people. On its own it is harmless, but in a baby it can be a clue to a hidden cleft of the palate.
1–2% of people
Body Rare ability
CCR5-delta32
About 1% of people of Northern European descent carry two copies of a broken gene that makes them almost immune to the most common form of HIV. A bone marrow transplant from one such donor produced the first cure of HIV.
About 1% of Northern Europeans (two copies)
Bones Variation
Cervical rib
An extra rib above the normal top rib, growing from the lowest bone in the neck. About 1 in 100 people have one, and most never know.
About 1% of people
Body Rare ability
Chimerism
A chimera is one person made of two sets of cells with different DNA. It happens when two embryos fuse in the first days of pregnancy. Most chimeras never find out. A few discovered it when a DNA test said they were not the mother of their own children.
Unknown. About 100 documented cases
Bones Variation
Clinodactyly
A finger that curves sideways toward its neighbours, most often the little finger leaning toward the ring finger. Between 1% and 20% of people have it, and on its own it is harmless.
1–20% of people
Mouth Trick
Cloverleaf tongue
Folding the tongue into three lobes, like a clover leaf. Between 10% and 15% of people in Western samples can do it. It is usually called the hardest tongue shape.
10–15% (Western samples)
Nerves Rare ability
Congenital insensitivity to pain
A tiny number of people are born unable to feel pain. Single-gene changes switch off their pain nerves. It is not a superpower. Injuries go unnoticed, and most people with it are badly hurt in childhood.
About 1 in 1,000,000
Ears Variation
Darwin's tubercle
A small bump on the folded rim of the ear, which Darwin thought was the leftover tip of a pointed ancestral ear. Between 10% and 58% of people have one, depending on the population.
10–58% of people
Eyes Variation
Distichiasis
A second row of eyelashes growing from the oil-gland openings along the inner edge of the eyelid. It is rare, mostly inherited, and Elizabeth Taylor was born with it.
Rare
Ears Trick
Ear wiggling
Moving the outer ears using three small muscles that most mammals use to point their ears at sounds. Between 10% and 22% of people can wiggle at least one ear. Many more can learn.
10–22% (one ear)
Mouth Variation
Geographic tongue
Smooth red patches with pale borders that form a map on the tongue and move around over days and weeks. About 1 to 3% of people have it, and it is harmless.
1–3% of people
Mouth Trick
Gorlin's sign
Touching the tip of your nose with your tongue. About 10% of people can do it. About half of people with Ehlers-Danlos syndrome can, so doctors use it as a quick check for stretchy connective tissue.
~10% of people
Eyes Variation
Heterochromia
Eyes of two different colours, or one eye with two colours, in fewer than 1 in 100 people. When it is present from birth it is almost always harmless.
Under 1% of people
Bones Variation
Hitchhiker's thumb
A thumb whose top joint bends back past 50 degrees, once taught as a simple recessive gene. Thumb angle is really a smooth range, and the genetics are not that simple.
24–36% of people
Bones Variation
Joint hypermobility
About 1 in 5 people have joints that bend further than usual, measured with a nine-point test called the Beighton score. For most people it causes no trouble.
10–25% of people
Bones Variation
Morton's toe
A second toe that looks longer than the big toe because the first metatarsal bone in the foot is short. It is common, and it is the foot shape seen on classical Greek statues.
3–22% of people
Muscle Variation
Palmaris longus
A thin tendon that stands out in the wrist when you press your thumb and little finger together. About 1 in 7 people do not have it at all, and they lose nothing.
Missing in about 15% of people
Brain Rare ability
Perfect pitch
About 1 in 10,000 people can name or sing any musical note without hearing a reference note first. It runs in families and is far more common in people who started music lessons very young.
About 1 in 10,000 people
Nerves Reflex
Photic sneeze reflex
About one person in four sneezes when they step into bright sunlight. It runs in families, and it is probably a wiring mix-up between the nerve for the eyes and the nerve for the nose.
About 18–35% of people
Ears Variation
Preauricular pit
A tiny hole in the skin just in front of the ear, present from birth, that leads to a short blind tunnel. It is harmless unless it gets infected.
0.1–10% of people
Brain Rare ability
Short sleeper syndrome
A small number of people need only 4 to 6 hours of sleep a night, wake without an alarm, and feel fine. Several gene variants that cause it have been found. Most people who sleep that little are simply sleep-deprived.
Well under 1% of people
Body Variation
Situs inversus
All the organs in the chest and belly sit on the opposite side from usual, as in a mirror. About 1 in 10,000 people are built this way, and most are healthy.
About 1 in 10,000 people
Skin Variation
Supernumerary nipples
An extra nipple, usually small and flat, on a line running from the armpit to the groin. Between 1 in 500 and 1 in 18 people have one, and most think it is a mole.
0.2–5.6% of people
Mouth Rare ability
Supertasters
About a quarter of people taste bitter compounds far more strongly than everyone else. They have more taste buds and a particular version of one gene, and they often dislike coffee, dark greens, and grapefruit.
About 25% of people
Brain Perception
Synesthesia
In some people, one sense triggers another. Letters have colours, sounds have shapes, or days of the week sit in fixed places in space. The pairings are automatic and stay the same for life.
About 4% of people
Eyes Perception
Tetrachromacy
Most people have three kinds of colour-sensing cone. A few people have four. In theory they can tell apart colours that look identical to everyone else. Confirmed cases are extremely rare, and online tests cannot detect it.
A handful of confirmed cases
Mouth Trick
Tongue rolling
Between 65% and 84% of people can curl the sides of the tongue up into a tube. For decades it was taught as a simple one-gene trait. It is not.
65–84% of people
Mouth Variation
Tongue tie
A short or tight band of tissue under the tongue that holds the tip down, present in about 1 in 20 babies. Some cannot breastfeed well, but many grow up with no trouble at all.
About 5% of babies
Bones Variation
Torus palatinus
A hard bony lump along the midline of the roof of the mouth that grows slowly and is completely harmless. Many people find it by chance and worry that it is a tumour.
About 20% of people
Bones Trick
Voluntary joint dislocation
Pushing a joint partly or fully out of its socket on purpose, then back in. About 20% of people have some joint hypermobility. True voluntary dislocation is much rarer and often linked to a connective tissue disorder.
Rare; no formal count Icky
Eyes Trick
Voluntary nystagmus
Between 5% and 8% of people can make both eyes shake rapidly from side to side on command. It looks alarming and is harmless. It is tiring and lasts only seconds.
5–8% of people
Bones Variation
Webbed toes
Two toes joined by skin, most often the second and third, in about 1 in 2,000 to 3,000 babies. The toes usually work normally and many people leave them as they are.
About 1 in 2,000–3,000 births