Genetics, development, and things that involve the whole organism.
Showing 12 oddities.
Some traits do not belong to one organ. They come from how the whole body is built, from a single gene, or from an event very early in development.
Situs inversus places every organ in the chest and abdomen on the mirror-image side from usual. Chimerism means one person carries two different sets of DNA, from two embryos that fused before birth. CCR5-delta32 is a single broken gene, carried by some people, that changes how their immune system responds to one virus.
These entries do not fit neatly into a single body part, because they are not really about one part. They are about a decision made once, early on, that then shapes the whole body for life. Most of them cause no trouble at all.
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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.
A chimera is one person made of two sets of cells with different DNA. It can happen 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.
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.
These entries belong to another region first, and touch this one too.
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.
A widening of the gap between the two halves of the six-pack muscle, seen in nearly every first pregnancy and in some men. A bulge down the midline when you sit up is the usual 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.
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.
A foot that bends in the middle, between the heel and the ball, as well as at the toes. About 1 in 12 people have it. It looks like the midfoot bend of apes.
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 have been linked to it. Most people who sleep that little are probably sleep-deprived.
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 it is often mistaken for a mole.
Most people have three kinds of colour-sensing cone. About 12% of women carry a fourth. In theory they can tell apart colours that look identical to everyone else. Clear cases are extremely rare, and online tests cannot detect it.
Pushing a joint partly or fully out of its socket on purpose, then back in. Between 10% and 25% of people have some joint hypermobility. One survey of students and their siblings found 0.5% to 2.6% could dislocate a shoulder at will.