Scientists have revealed details of the world's only known case of "semi-identical" twins.
The journal Nature says the twins are identical on their mother's side, but share only half their genes on their father's side.
They are the result of two sperm cells fertilising a single egg, which then divided to form two embryos - and each sperm contributed genes to each child.
Each stage is unlikely, and scientists believe the twins are probably unique.
Whether these things are academic curiosities, or whether we've overlooked something significant is hard to say
snippy.
Normally, twins either develop from the same egg which later splits to form identical twins - who share all their genetic material, or from two separate eggs which are fertilised by two separate sperm.
This creates non-identical (fraternal) twins - who share 50% of genetic material.
Sometimes, two sperm can fertilise a single egg, but this is only thought to happen in about 1% of human conceptions.
Most embryos created this way do not survive.
Hermaphrodite
These twins, who were conceived normally, only came to the attention of scientists because one was born with sexually ambiguous genitalia.
The child was discovered to be a hermaphrodite, and has both ovarian and testicular tissue, while the other child is anatomically male.
But genetic tests show both are "chimeras", and have some male cells - which have an X and Y chromosome, and female cells - which have two X chromosomes.
The most likely explanation for how they were formed is that two sperm cells - one with an X chromosome and one with a Y chromosome - fused with a single egg.
