Study reports rare semi-identical twins born

Twin infants usually develop in one of two ways: either a single fertilized egg becomes two separate embryos that grow (producing identical twins), or two separate fertilized eggs grow side-by-side in utero (fraternal twins). A study published in the April 2007 issue of the journal Human Genetics reports a rare set of twins who did not develop in either of the usual ways. These twins developed when a single egg from the mother was fertilized by two sperm from the father. The fertilized egg then split and became two separate embryos that developed into infants. The infants are identical in terms of their mother's genes but have different amounts of their father's genes. One child was born with both male and female reproductive tissue, while the other child only has male reproductive organs. News@nature.com reports that the children, now toddlers, appear to be growing and developing normally.

Source: Souter, Vivienne, Melissa A. Parisi, Dale R. Nyholt, Raj P. Kapur, Anjali K. Henders, Kent E. Opheim, Daniel F. Gunther, Michael E. Mitchell, Ian A. Glass, & Grant W. Montgomery. "A case of true hermaphroditism reveals an unusual mechanism of twinning." Human Genetics 121(2007): 179-185.