Ananova:

Doctors have discovered the world's first known case of 'semi-identical' twins.

The two young children in the US are identical on their mother's side but share only half the genes of their father's side.

They are the result of two sperm cells fertilising a single egg, which then divided to form two embryos, reports the Daily Mail.

Normally twins develop from one egg which splits early in development, creating identical twins, or they are the product of two separate eggs fertilized by two different sperm, creating non-identical twins.

The new case came to light because one of the twins had an abnormality in sexual development with both ovarian and testicular tissue. This child is being raised as a girl. The other twin is a boy.

Writing in the Journal of Human Genetics, researchers said the "semi-identical" twins are more genetically similar than fraternal twins, but less similar than identical twins.

"This shows that our understanding of how twinning arises is probably something of a simplification, and that there are some very probably rare variations on how this can arise," lead author Dr Vivienne Souter said.

The two children are doing well according to co-author Dr Melissa Parisi: "I can tell you that in my last contact with the family, the twins were doing very well - healthy, growing well, developing normally."