One twin's liver may be on the right while the other's sits on the left. Some twins are born as mirror twins, which is when one twin has physical features that are the mirror image of those of the other twin. This can be in the form of the layout of moles, the direction in which their hair curls, or even the layout of their internal organs. It’s thought to occur when an embryo splits in two, as we observe in “identical twins” (though not everybody agrees with this terminology). While mirror twinning isn’t itself a scientifically recognized group, it’s a term used to describe a subset of twins whose features are almost identical, but asymmetrical. How do twins happen? Mirror image identical twinning can occur in any pregnancy with “identical twins”, which are produced as the result of a single egg diving into two embryos, also known as monozygotic twins. The twins will have the same genes so will be the same sex and look very similar, but geneticists often reject the “identical” label owing to the fact that having the same genes doesn’t mean you will look identical (and monozygotic twins aren't always genetically identical either). Dizygotic twins are born as the result of two eggs being fertilized by different sperm at roughly the same time. These fraternal twins, as they’re also known, will have different genes so can be different sexes and look different from one another. Mirror twins Mirror twins form through the same process as identical twins with a single egg dividing into two monozygotic twins. It’s theorized that the asymmetrical physical traits are the result of the division happening later in the embryo’s development – more than a week after conception – with the most extreme mirror twinning seen in conjoined twins. The result in mirror twins is two asymmetrical twin babies who may be almost identical except for the fact that one twin is the reverse of the other. They get their name for the fact that when facing each other, they see near enough what they would see in a mirror. Features that can be flipped in mirror twins (but aren’t always) include left- or right-handedness, the arrangement of moles, birthmarks, eyebrow shape, clockwise or counter-clockwise hair whorls, dentition, and even, albeit rarely, organ arrangement, something that’s known as situs inversus. They can also have different preferred sides to sleep on, something that can affect health outcomes as left- and right-sided sleeping can open you up to different health problems. The degree of mirror imaging for mirror twins can vary depending on whether traits are decided by their genes or epigenetics – processes that act on our DNA to alter the way it’s expressed. Mirror twins’ mirrored health conditions As well as mirroring each other’s appearances, mirror twins can sometimes develop the same health conditions, which are also flipped. One such example is the case of two mirror image twins who developed the same kind of lung cancer in mirror-image locations. There have also been case reports of mirror twins developing brain cysts and eye conditions in mirror-image locations. Twins were recently the focus of a Gucci runway campaign. Can you spot any mirror twins among them? Source
