We all know what twins are; normally you can have identical twins or fraternal twins. Identical twins are the same sex and have the same DNA, fraternal twins do not. There is another type of twin you may have never heard of because of the rarity, they are called semi-identical twins.
For only the second time in history, semi-identical twins have been born, and this is the first time that they were identified during pregnancy. The twins are from Queensland, Australia and genetic testing revealed they were identical on the mother’s side, but only have part of their father’s DNA.
These twins are now four years old, and while they have 100% of their mother’s genes, the two are said to be like normal siblings on their father’s side. As for how the rare pregnancy happened, the medical staff that cared for the mother during pregnancy say that it’s likely that the mother’s egg was fertilized simultaneously by two of the father’s sperm, before dividing.
The mom’s first ultrasound showed a single placenta and amniotic sacs indicating identical twins. A later ultrasound showed that the twins were male and female, which is impossible in normal identical twins. Scientists say that embryos resulting from fertilization from two sperm don’t typically survive.
In this case, the egg was able to divide up the three sets of chromosomes into cells that then split into two eggs resulting in twins. The twins are healthy and have hit all developmental milestones. The twins came to the researcher’s attention after an investigation revealed the mixed chromosomes.