New spin on genetics: Semi-identical twins

[one...two...tree 2,762]

John Faherty

The Arizona Republic

Twins used to be so simple. They were identical or fraternal.

Not anymore.

Vivienne Souter, a doctor at Banner Good Samaritan Medical Center in Phoenix, is the lead researcher in the discovery of a new type of human being: the semi-identical twin.

To call this type of twins rare is a gross understatement. Before Souter's report today in the Journal of Human Genetics, this type existed only as a theoretical possibility.

Souter said it did not take long to realize the twins studied were different from any other pair.

"Right from the start, we knew these were very unusual twins," she said. "But when we saw the results of the genetic testing, we realized they were unique."

In identical twins, a single sperm fertilizes a single egg, which then splits, resulting in two children exactly alike.

With fraternal twins, there are two eggs simultaneously fertilized by two sperm, resulting in two genetically distinct children.

This new pair of twins, whose identities are being kept secret, came to be differently.

One possibility is that one egg was fertilized by two sperm before splitting apart.

The second option is that a single egg cleaved before being fertilized by two sperm.

Either way, the twins are identical on their mother's side but not on their father's.

They represent a point somewhere between identical and fraternal.

It will be years before we know what this discovery will mean for research, Souter said. But it is certain to have meaning.

"Whether these things are academic curiosities, or whether we've overlooked something significant is hard to say," biologist Michael Golubovsky of the University of California-Berkeley told the Web site Nature.com. "We need to keep our eyes open for other unusual scenarios."

The children, who were born in the United State and are now older than toddler age, came to the attention of Souter, herself the mother of twins, and her fellow researchers because one of them is considered a true hermaphrodite. The other is male.

In clinical language, the genitalia of a hermaphrodite are described as ambiguous, meaning the child has both ovarian and testicular tissue.

That condition led to extensive genetic testing, which revealed the unique nature of the children's conception.

In addition to their status as semi-identical twins and with one child being a hermaphrodite, the children have another interesting feature.

Both are chimeric, which means each carries two sets of genetic blueprints, while nearly every other person has only one. The word "chimeric" comes from Greek mythology. The Chimera, a mythological creature, had the head of a lion, the body of a goat and the tail of a serpent.

Souter, 41, says both children are doing well. She will not say where they live but said they are growing healthy and strong.

She has studied genetics and twins for much of her professional life, so it seemed fitting nearly four years ago when she had a set of her own.

"Absolutely, being the mother of multiples is definitely different from singletons," Souter said. "They have a very special bond, twins. Anything I can do to help increase understanding is good."

This week, she finished her residency in the department of obstetrics and gynecology at Good Samaritan.

She says the field of medical genetics is becoming more exciting as technological advances lead to greater understanding of who we are and how we become who we are.

"It is amazing what is happening right now," Souter said. "I think genetics have just opened up a huge opportunity to look into biology like never before."

[Jenn! 918]

I think Mary-Kate and Ashley must be semi-identical.

[one...two...tree 2,762]

I think Mary-Kate and Ashley must be semi-identical.

I could never tell them apart and for awhile I thought there were 3 of them...Mary, Kate and Ashley.

[almaty 19]

I think Mary-Kate and Ashley must be semi-identical.

I could never tell them apart and for awhile I thought there were 3 of them...Mary, Kate and Ashley.

same here..did not know there were only 2 ..thnaks for the up-date