Two four year old siblings in Brisbane Australia have become only the second case of being semi-identical twins.

The sister and brother share all their mother’s DNA but only a section of their father’s as reported in the New England Journal of Medicine.

Genetic testing was used to first identify that the two were sesquizygotic.

According to Professor Nicholas Fisk, who looked after the mother and semi-idenitcal twins at Royal Brisbane & Women’s Hospital said:-

“It is likely the mother’s egg was fertilised simultaneously by two of the father’s sperm before dividing,” he said.

“The mother’s ultrasound at six weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins.

“However, an ultrasound at 14 weeks showed the twins were male and female, which is not possible for identical twins.”

Read more on The Queensland University of Technology Website