G'day team,

Today I thought I'd have a chat about one of the coolest little bits of human biology. Grown men and women with two distinct genomes, who exhibit no signs of their unique biological makeup to the outside world.

So what is a chimera?

The Chimera

Let me introduce... the Chimera!

The legenderay Chimera (baby version)
Image source

Now, this Chimera is the ancient Greek monster that ravaged Lycia, but it's after this beast that the modern day human chimeras are named. With the head and body of a lion, a goat's head and udders and a serpent for a tail the Greek chimera was a beastly fusion of creatures. It's after this concept of fusion (not of lions, goats, and serpents) that the Human Chimera is named...

The Human Chimera

A human chimera is a hybrid human, whose body contains two completely different sets of DNA. There are a number of ways that humans can become a chimera, and a number of different levels of 'fusion', so let's have a look at a few.

These human-human fusions can occur at many points in our lives, but by far the most awesome is the teragametic chimera... a complete fusion of two separate beings starting in the earliest stages of development.

Tetragametic Chimera

Tetragametic chimeras are the result of a fusion of two non-identical embryos in utero... in English, this means that two eggs are fertilized by two sperm but become one person. Somewhere during development (quite early on) the small bundle of cells that are destined to become grown humans fuse and begin to grow together. From then on the cells will intermingle and grow in unison to create a single grown baby made from two distinct genomes!

Now the 'intermingling' of tetragametic chimera may not be completely homogenous (mixed-up) and the resulting 'patchwork' style body can have some interesting outcomes. For example, a whole liver may belong to one cell-line with one set of DNA, while an entire liver is from the other cell line with its own unique set of DNA. One eye may be blue, while the other is green, or the colour split may occur within the same eye (called heterochromia)! This can result in some fairly unique features in these individuals. Tetragametic chimera may have differences in hair colour and growth across the body, differences in skin tone or even a combination of a normal and a hitchhikers thumb.

A Heterochromic Eye
Image source

Now having two unique sets of DNA has some crazy implications throughout life... Giving blood can produce some interesting results, as you'll have two separate blood types. If you have children you may be able to produce offspring from either two sets of DNA, so you may have some surprising variability among your kids!

And there have some really horrible mix-ups in history due to this confusing condition. In 2002 a woman named Lydia Fairchild underwent some standard litigations and tests to get child support from a recently divorced husband. The result of these tests found that yes, the man was her children's father, but she was not their mother. She was accused of fraud and the state sued to confiscate her children. It was during this trial that the prosecution discovered that Fiarchild was a genetic chimera and the case was settled.

Now, this is an extreme version of human chimerism, but there are other ways that our bodies can integrate another set of DNA into our system that are seen far more regularly...

Blood Chimera

Blood chimera is people who're biologically a single person, but whose blood contains cells (with which they're compatible) from another human. The 'other' set of blood cells usually originates from the fetal blood of a twin, shared during development.

Monochorionic monoamniotic twins, sharing a placenta and amniotic sac
Image source

The confusion arises when someone is found to be a blood chimera but has never had a twin! If this person's second set of blood DNA does not match their mothers, where could it have come from?... This sad answer to this puzzel is simple. Not all fertilized eggs survive through development. A set of twins may form a shared blood supply, and then one may perish before it's even large enough to detect. In some cases, a more developed twin may also absorb it's womb-buddy, a phenomenon called vanishing twin syndrome. It has been suggested that up to 8% of non-identical twins are blood chimera.

Now, these are the last of the 'true' chimera, but there's one last class of chimera that just slips into the description.

Microchimera

Many people are microchimera and have cells in their bodies that are not their own, but also don't form part of their constitutional makeup. This means that eventually these cells will die and the separate DNA will disappear too. There are a number of ways to become a microchimera including receiving a blood transfusion, an intermingling of fetal and mothers blood during pregnancy or on rare occasions the reabsortion of a fetus by a pregnant women.

Thanks

Thanks team, I hope you enjoyed reading and learned something interesting! As always please look below for more medicine-related posts and comment any questions or comments you may have!

Thanks team

-tfc

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Resources

• MedicineNet
• Chimerism and teratagamic chimerism in humans: implications in autoimmunity, allorecognition and tolerance

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