Study describes new type of "semi-identical" twins

WASHINGTON (Reuters) - Doctors said on Monday they have identified a third type of twins -- somewhere between identical and fraternal -- after performing extensive genetic tests on two young children.

They are referring to the pair as “semi-identical” -- two sperm cells fused with a single egg -- and said this is a previously unknown way for twins to arise.

With fraternal twins, the most common type, the mother contributes two eggs that each are fertilized in the womb by two different sperm cells from the father. They are genetically as similar as any ordinary siblings.

With identical twins, one egg from the mother is fertilized by one sperm from the father, and then very early in development the embryo splits and two fetuses grow. These twins are very similar genetically.

The new case came to light because one of the twins had an abnormality in sexual development -- ambiguous genitalia -- and was considered a hermaphrodite with both ovarian and testicular tissue. This child is being raised as a girl. The other twin is a boy.

Writing in the Journal of Human Genetics, researchers said the “semi-identical” twins are more genetically similar than fraternal twins, but less similar than identical twins. “This observation suggests the existence of other similar twins that have not yet been, and may never be, identified,” they wrote.

“I think the most important thing is that this shows that our understanding of how twinning arises is probably something of a simplification, and that there are some very probably rare variations on how this can arise,” Dr. Vivienne Souter of Banner Good Samaritan Medical Center in Phoenix, lead author of the study describing the twins, said in an interview.

“Since we don’t ordinarily look at the genetics of twins, there probably are more cases out there, but they have not come to light because they haven’t been investigated in the way that these twins were,” added Souter, a geneticist also trained in obstetrics and gynecology who is the mother of fraternal twins.

The “semi-identical” twins were evaluated at a U.S. hospital initially a few years ago, the researchers said.

Dr. Melissa Parisi, a pediatrics professor at University of Washington in Seattle and co-author of the research, said, “I can tell you that in my last contact with the family, the twins were doing very well -- healthy, growing well, developing normally.”

Souter said the twins were conceived completely naturally, noting that in vitro fertilization or infertility treatments were not involved.

Some experts, including biologist Michael Golubovsky, theorized that such twins might exist, and these twins are the proof. The study was first reported on by, the online site of the journal Nature.