Take any two cells from your body and there's a good chance their genetic sequences will be a match. That is, unless you happen to have what's referred to in the medical literature as a 'tetragametic chimerism' – a condition that causes separate fertilised embryos to merge into a single body.
Once thought to be rare among humans, there's good reason to suspect we might be seeing a lot more of it in the future.
The truth is, nobody is really certain how many humans have cells in their body that once belonged to a sibling.
Unlike the transplantation of a fully developed organ into a grown body, the presence of genetically distinct cells early in life won't trigger alarm bells in the immune system.
That makes it possible for two (or more) fertilised eggs – siblings, for all purposes – to bump into one another in the uterus, and mistake their new neighbour for another cell in their own growing body. Far from rejecting each other, the cells team up and continue to build a single human.
The tides of hormones and other chemical signals directing the embryo's evolution tend to affect the two cells in pretty much the same way, making it hard to spot where one developing tissue ends and another begins.
On occasion, however, the different sets of genes do make themselves known.
Increased reliance on assisted reproductive technologies means we can expect more multiple conceptions.
Just how many of these embryonic siblings merge into a single person is far from clear. Given how little we know about either its prevalence or how it might be responsible for anything from allergies to auto-immune disorders, this could be concerning.
But no doubt it's a topic we'll be learning more about in coming years.
Petrina Duncan, Obstetrician, Gynaecologist, and mother. Writes about events, news issues and health issues.