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 eggan event
assumed to be very rarethen 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 [email protected], 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.
any twins here? 
