Rare new kind of twins: boy and girl are semi-identical

Doctors in Australia say they have identified a second case of twins apparently created from one egg and two sperm, a boy-girl combination in whom the mother’s DNA is identical in both babies but the father’s DNA varies in each twin. Virtually all twins are either fraternal (two eggs and two sperm have created two separate embryos) or identical (one sperm and one egg create two separate embryos through a division at the blastocyst stage, before the formation of the amniotic sac). In the case of the Australian twins, who live in Brisbane and are now 4½ years old, the mother’s egg was fertilized with one sperm carrying an X chromosome and one with a Y. Because an ultrasound taken early in the pregnancy showed both fetuses shared the same placenta, doctors assumed the fetuses were identical twins.

Source: www.cbc.ca