An article in the NY Times discusses the discovery and occurrence of neither fraternal nor identical twins, but semi-identical twins; also referred to ask sesquizcgotic twins. The twins share 100% of their maternal DNA and overlap on portions of their paternal DNA. Therefore, the twins are approximately three-fourths identical.
At 14 weeks, Dr. Nicholas Fisk, a maternal fetal medicine specialist at the Royal Brisbane and Women’s Hospital, inspected an ultrasound and observed one body and one girl, sharing a placenta. Normally, identical twins develop when a single egg is fertilized by a single sperm and splits into two. However, regarding the semi-identical twins, Dr. Fisk suspects that a single egg was fertilized by two sperm before dividing because when the initial sperm entered the egg, the membrane failed to lock down to stop any other sperm from entering. Although, even if another sperm got into an egg, typically, it would result in three sets of chromosomes and a fatal outcome but this time it did not.
Therefore, Dr. Fisk and his colleagues conducted an analysis of every single chromosome in the boy and the girl. They found no negative abnormalities, the mother had two healthy babies. Each baby contained the normal number of chromosomes, however, there were some areas where their DNA was identical and others where it was different, hence the term "semi-identical" twins. The children were monitored through out their childhood. By age 4, they were meeting all of their developmental markers by the appropriate time. Dr. Fisk sought to determine whether this phenomenon was more common than previously thought. He reviewed genetic data from nearly 1,000 fraternal twins but did not find any semi-identical twins among them.
The discovery of the concept of semi-identical twins bring about a new phenomenon regarding the way in which mammalian eggs can be fertilized. More research needs to be conducted in order to find out the exact process by which this occurrence is possible.