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 The
Strange Truth About Twins
by
Martha Brockenbrough
Originally Published
Here
I was discussing important matters recently with
a friend who happens to have a Ph.D in pharmacology, and she
told me something that really rattled my molecules: The Olsen
twins, Mary-Kate and Ashley, aren't actually identical--no
matter how much they resemble each other on TV.
Like, oh no, I thought. How could it be? This revelation of the
perky twosome's nonidentical nature is almost as hard for me to
believe as the vastness of the teen actresses' billion-dollar
fashion and entertainment empire.
I felt a burning curiosity to know whether the statement was
true, and yet I could not bring myself to track down their
mother and ask. So naturally, I searched the Internet instead.
My search generated a
host of results. I poked around a bit, and though I did locate
information about the "mary-kateandashley brand" of jeans, I
couldn't find much information about the kind of genes I was
seeking.
And while I did find some sites that reported that the twins are
fraternal, these sites didn't back that up with anything beyond
saying that one is left-handed and one is right-handed.
As I have since learned from Nancy L. Segal, a professor of
psychology at California State University, Fullerton, this
doesn't prove a thing. Twins can be identical and yet use
opposite hands for writing. Blood tests can generally tell you
for sure whether twins are identical, though sometimes you need
to look at DNA markers to ferret out the truth.
To really know whether Mary-Kate and Ashley are fraternal or
identical, we'd need a little of their blood. But alas, that's
one of the few things they're not selling on their Web site.
Fortunately, by this time I was finding myself more interested
in the mysteries of twins in general than in the details of this
specific set.
Did you know that every August twins from around the globe
descend upon Twinsburg, Ohio, for the annual Twins Days
Festival. In the past, participants have included twin nuns
named Celeste and Celestine, as well as Craig and Mark Sanders
and their wives, Darlene and Diane. Craig and Mark are twins, as
are Darlene and Diane. They met at the festival four years ago,
married, and now live next door to each other in Houston, Texas.
Craig and Diane are now the parents of twin boys. If it weren't
for the fact that Mark and Darlene have a single daughter, their
neighbors would probably think they were seeing double.
But I digress. Why are twins so fascinating, anyway?
Part II: Why study twins?
Long before professors started studying the blood, bodies, and
brains of twins, literature and history celebrated them.
Romulus and Remus get credit for being the twin founders of
Rome. Shakespeare, who had opposite-sex twin children named
Judith and Hamnet, wrote The Comedy of Errors and Twelfth Night,
two plays about twins and mistaken identity. The plot of Charles
Dickens's A Tale of Two Cities also revolves around look-alikes,
as does Mark Twain's The Prince and the Pauper. Star Trek
popularized the classic evil-twin-with-goatee motif when Spock
got a doppelganger.
For ages, people have thought twins were cool. With modern
research, though, we can have an even deeper appreciation.
Because twins share as much as 100 percent of their genetic
material, we can use them to understand better why we are who we
are, even if we aren't twins. It's like getting a window onto a
parallel universe.
While being an identical twin doesn't mean that you have a
carbon copy running around, identical twins are remarkably
similar.
The Minnesota Study of Twins Reared Apart studied one famous set
known as "the Jim twins." Both boys were named Jim. Both married
multiple times, and curiously, the first wife of each was named
Linda and the second was named Betty. The twins named their sons
James, and they both as children had dogs named Toy. They even
vacationed at the same Florida beach, each driving there in a
light blue Chevrolet--although their paths didn't cross until
later.
Coincidences like this suggest very strongly that a lot of
choices we make are inspired by our genes. (However, Segal says
that scientists, unfortunately, will never be able to isolate
the blue Chevrolet gene sequence.)
Fraternal twins share less genetic material than identical
twins--but they share more than two unrelated people do. They
also help investigators make comparisons, according to Segal,
who wrote Entwined Lives: Twins and What They Tell Us About
Human Behavior.
Among the many interesting ways we can learn about fraternal
twins is to measure whether sets with higher proportions of
common genes (such as the Olsen twins) resemble each other more
than those with less overlap.
One key insight Segal reports is that researchers, by comparing
identical and fraternal twins, have found identical twins more
likely to have comparable intellects than fraternal twins--solid
evidence that genes have a substantial influence on
intelligence.
But it gets even more interesting. As it happens, identical
twins can be shockingly different. And in rare cases, fraternal
twins might not even have the same father.
Part III: Twins: More than just identical and
fraternal
Twins are born when a single egg that is
fertilized splits, or when multiple eggs are fertilized by
separate sperm. The single-egg infants are identical twins,
while the multiple-egg offspring are fraternal. Minivans get
even more crowded when those eggs split three ways, or when
three eggs are fertilized. That makes triplets. I don't even
want to think about quadruplets and beyond, but the process is
the same no matter how high the diaper count rises.
This is pretty basic knowledge nowadays. What may not be,
however, is that there are four types of identical twins. They
are categorized based on whether they have their own or a shared
placenta, chorion, or amnion (the latter two are membranes
surrounding the fetus). The earlier the eggs split, the more
independent equipment the fetuses get. For example, if the egg
divides before the chorion forms by day five after conception,
then each twin will get its own. But if it divides later, the
twins will share. Researchers at Indiana University have
discovered that the later the split (and thus the more that is
shared), the more similar the twins' personalities will be.
Fraternal twins, usually the second-fiddle in the multiple-birth
orchestra, actually can be more interesting to contemplate. For
example, fraternal twins can be conceived days apart, if a woman
releases eggs on different days during the same menstrual cycle.
Even more dramatic, some women who are pregnant ovulate during
their pregnancy and get pregnant with a second baby, then
deliver full-term infants at different times, or deliver one
full-term and one premature baby at the same time.
Granted, this is but a tiny percentage of fraternal twins (who
are already relatively rare, at one birth in 125). The only way
to make it seem halfway normal is to ponder the fact that
nine-banded armadillos always give birth to identical
quadruplets.
And
yet, even this is not the most remarkable thing that twin
researchers have discovered. That honor goes to the realization
that identical twins are not necessarily always of the same sex.
I know; it seems impossible. Everyone knows boys and girls have
different parts and are therefore clearly not identical, right?
Not quite. Girls have XX chromosomes and boys have XY
chromosomes. The egg cell is always an X, and the sperm cell is
either an X or a Y. If the Y chromosome is lost during the early
stages of development, the child is born as a female with
something called Turner Syndrome. Girls with Turner Syndrome are
generally short, can't have children, and may have weak spatial
skills. Although they are XO and not XX, they're still girls,
and those with brothers who arose from the same egg do have the
identical genetic material.
Given this, it would be nice to be able to count on some old
myths about twins, such as the one that says fraternal twins
skip a generation. But this isn't true either, Segal says.
Mary-Kate and Ashley Olsen might, in the distant future, give
birth to twins of their own. If they meet their future husbands
at Twins Day, and those men also are twins, then hey--they would
be practically giving birth to each other's children. That's
what happens when identical twins marry identical twins and have
children. Their nieces and nephews are, genetically, their own
kids.
Unless Mary-Kate and Ashley take a DNA test, which Segal thinks
they should do for the sake of all their young fans, we have no
way of knowing whether they could be the genetic parents or
half-parents of each other's children. For the record,
Segal--who has been known to identify identical twins without
benefit of blood test 94 percent of the time--thinks Mary-Kate
and Ashley are indeed identical.
And so, the controversy rages on. Just thinking about it is
enough to make me glad I didn't bother their mother, though.
Something tells me she's going to have her hands full for a long
time.
Martha Brockenbrough lives, writes, and plays in Seattle.
She is author of
It Could Happen to You: Diary of a Pregnancy and Beyond.
Want to Learn More?
Check out the
official
Twins Days site.
Where is
Twinsburg?
Meet the
Sanders families (and other amazing twins clans).
Read
Entwined Lives: Twins and What They Tell Us About Human
Behavior by Nancy L. Segal.
Read Twain's classic
The Prince and the Pauper.
Advance twins research by participating in one of the following
studies by professor Nancy Segal:
For
twins
who are parents.
For
unrelated
siblings of the same age who were reared together since
birth.
You can test your
Twin DNA here.
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