Affecting as few as 1 in 20,000 people is a genetic condition known as situs inversus, where the internal organs are positioned in a mirror image to what's considered normal. This doesn't always happen with all of the organs at once: sometimes, it's just the heart that's flipped to the other side; other times several organs are flipped while others aren't. By itself, situs inversus doesn't usually cause harm, but it can come coupled with various other genetic conditions that can cause heart problems and other medical issues. The chances of a child being born with the condition are strikingly rare, but it does also happen with "mirror image" twins whose fertilized embryo splits late in its development.
If beauty lies in symmetry, a human's beauty is truly skin deep. That's because when you look past our visible features and delve inside our bodies, symmetry is the exception to the rule: your stomach and heart are on the left side, your liver is on the right, and your right lung has more lobes than your left -- for most people, that is.
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On the outside, we look symmetrical, but that stops with our internal organs. Even organs that look symmetrical at first tip one way or the other -- the right lung is divided into three lobes while the left only has two, for instance. 00:12
We all have the same internal asymmetry, except for the 1 in 20,000 people who have a condition known as situs inversus, where the internal organs are inverted left to right. This usually causes no negative effects, except for rare emergency cases when it can keep doctors from identifying which organ is in pain. 01:11
Researchers looking at a special bunch of cells in a central region of an embryo found that its hair-like cilia began to beat in unison toward what would later become the left side. This beating washes a current over the outside of the embryo that activate pressure sensors, which turn on a gene that determines which side the embryo's organs will live on. 03:07