Sesquizygotic Twins: Second Set of Semi-identical Twins Discovered by Scientists

An extremely rare set of semi-identical twins has been identified in Australia—becoming only the second ever confirmed.

The "sesquizygotic" twins were identified by doctors in Brisbane while still in the womb. DNA analysis showed they were identical on the mother's side, but just siblings on the father's side. A case report on the twins, who are now four years old, has been published in the NEJM.

A Third Type of Twin

In the case of identical —known as monozygotic—twins, a single sperm fertilizes an egg, which then divides into two, producing babies that share the same DNA. In fraternal, or dizygotic, twins, two different eggs are fertilized by two different sperm. With semi-identical twins, an egg is fertilized by two sperm, forming a triploid, which then splits in two.

The first sesquizygotic twins were identified in 2007. According to a report in Nature magazine from the time, this "double fertilization" occurs in about 1 percent of human conceptions, but normally the embryo does not survive. The unusual genetic makeup of the twins was only discovered because one of them had ambiguous genitalia.

The latest set of semi-identical twins was found because of an unusual ultrasound—at six weeks, the scan showed a single placenta and amniotic sacs that indicated she was expecting identical twins. However, eight weeks later an ultrasound showed the twins were male and female, which is impossible for identical twins.

"It is likely the mother's egg was fertilized simultaneously by two of the father's sperm before dividing," Nicholas Fisk, a fetal medicine specialist who was involved in the mother's care, said in a statement.

Michael Gabbett, a clinical geneticist also involved in the case, added: "The fertilized egg appears to have equally divided up the three sets of chromosomes into groups of cells that then split into two, creating the twins. 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 100 percent of the same paternal DNA."

Hiding in Plain Sight?

After the first set of semi-identical twins was discovered, it was suggested that there may be many more. The problem is that unless they need to be genetically tested for something, they will remain hidden from science.

However, Fisk said this is not the case. In an analysis of a worldwide database of twins, he and his colleagues looked for other cases where sesquizygotic twins might have been wrongly identified as fraternal twins. They looked at the genetic data from 968 twins and their parents, and did not find a single case of semi-identical twins.

sperm egg semi-identical twins
The semi-identical twins came from two sperm fertilizing one egg. QUT
Sesquizygotic Twins: Second Set of Semi-identical Twins Discovered by Scientists | Health