Semi-Identical Twins Discovered

In the BBC:

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…

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.