Some people's blood contains cells from a sibling. Others are two individuals rolled into one. Yet more carry a distinct mutation in only parts of their bodies

These are human genetic oddities known as Chimaeras and Mosiacs.

Chimaeras are humans who are composed of two genetically distinct types of cells, caused by two different eggs (ie twins) fusing into one embryo. So a person can have some body parts which are male, and some which are female!

Mosaics have patches of tissue that differ genetically from the rest of their body, thanks to a mutation or chromosomal anomaly that arose early in embryological development.

The article looks at how many more of these people exist without being detected, and the problems of providing medication for someone who is really two different people.

And with the the use of IVF, comes more twins, and more chimaeras and mosiacs. More info...