In 99.9% of cases boy/girl twins are non-identical. However, in some extremely rare cases resulting from a genetic mutation, identical twins from an egg and sperm which began as male (XY) can develop into a male / female pair.  This happens when the fertilised egg 'loses' one of the copies of the Y chromosome when it is dividing into two embryos in very early development. The resulting babies are then male (XY) and female (XO). The normal genetic make-up of a girl is XX. An XO baby is outwardly a girl, but her cells only have one copy of the X chromosome. This condition is called Turner Syndrome (more information on this, see the following link: Turner Syndrome).