A set of twins, one boy and one girl, born in Australia are only the second documented case of semi-identical twins ever recorded. They were born in 2014, but their case study just appeared in The New England Journal of Medicine on Feb. 28.
Semi-identical twins result when two of the father’s sperm fertilize one of the mother’s eggs before it splits, resulting in three sets of chromosomes, one from the mother and two from the father.
In this case, the mother’s ultrasound at six weeks of gestation showed a single placenta, and positioning of the amniotic sacs indicated identical twins. But at 14 weeks a second ultrasound showed one male and one female baby, an impossibility for identical twins. Doctors determined that the fertilized egg equally divided the three sets of chromosomes into groups of cells that then split, creating two babies. “Some of the cells contain the chromosomes from the first sperm while the remaining cells contain chromosomes from the second sperm, resulting in the twins sharing only a proportion, rather [than] 100 percent, of the same paternal DNA,” Michael Gabbett, a physician on the medical team, said in a statement.
Physicians first reported a case of semi-identical twins in 2007, when doctors discovered the babies had ambiguous genitalia. DNA analysis showed that the twins, a boy and a girl, shared identical DNA from their mother but only about half of their father’s DNA. —J.B.