New Type of Twins Discovered
Twins can be identical, fraternal and apparently semi-identical, scientists now report. Researchers discovered twins who are identical on their mom’s side of the equation but share only half their genes from dad. Here’s how it happened: Two sperm cells fertilized one egg—an event assumed to be very rare—then split into two embryos.
“Their similarity is somewhere between identical and fraternal twins,” said geneticist Vivienne Souter, of the Banner Good Samaritan Medical Center in Phoenix, Arizona. “It makes me wonder whether the current classification of twins is an oversimplification.”
The finding, detailed in the Journal of Human Genetics, was reported today by news@nature.com, the online site of the journal Nature.
Identical twins are created when one fertilized egg splits into two embryos. They share the same placenta and are always of the same sex. Fraternal twins result from two eggs being fertilized at the same time, each by a different sperm. Each has its own placenta, and they can be the same sex or not.
The semi-identical twins only came to the attention of Souter and her colleagues because one had ambiguous genitalia. The child was born a “true hermaphrodite” with both ovarian and testicular tissue. The other twin is a male, anatomically.
The twins are now toddlers, according to the report. They were conceived and born normally and appear to be mentally normal and are growing normally.
My comments: Ok, maybe I’m missing something, but how do parents not notice if their child’s genitalia is ambiguous? Or the pediatrician?
Krysti, there are lots of causes of hermaphroditism (and they're not sure about them all), but it's often a hormonal thing - like the inability of a male's body (in the womb) to respond to testosterone - which results in the formation of some female organs.
Shan, it doesn't sound to me like your twin friends are identical, but maybe this semi-identical thing might explain them. I would think it's way more likely that they are fraternal but were assumed to be identical because perhaps they shared a placenta (or the 2 placentas fused together and appeared to be one).
I wonder how common this new "semi-identical" category is (they did say "very rare") and how it would present itself in genetic test results. Many parents of twins test their children to see if they're frat or ident, and it sounds like this new category would fit into one of those categories, or else they would have discovered it earlier? So do semi-identicals present as genetically identical? Or fraternal, so people don't think much of the physical differences because they think the twins are fraternal anyway? And how much testing is required to discover that they're really only semi-identical?
Whew! Lots to think about. Okay, turning my science brain off now.