Twins: And What They Tell Us About Who We Are (17 page)

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Authors: Lawrence Wright

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BOOK: Twins: And What They Tell Us About Who We Are
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the maternal line. And yet here was a young girl with all the marks of full-blown Duchenne, which in fact did continue its remorseless progression as Burn and his colleagues studied Jenny over the next several years, placing her in a wheelchair by the age of eleven and leaving her dead at sixteen. Why, Burn wondered, did this girl suffer from a disease that is supposed to be found only in boys? Even stranger, how was it that her identical twin sister was spared this appalling destiny?
John Burn had become interested in twins through his early work on heart defects at Great Ormond Street. "I was struck by the fact that such defects are a remarkably constant problem in time and space, suggesting that they were almost an intrinsic part of being as far as mankind was concerned. At the same time, if you looked in the textbooks, they all said that heart defects are not a genetic problem. Their evidence was based on twinsactually, like those two over there," he said, pointing to a photograph of blond identical twin boys atop his filing cabinet. "One of the twins has a severe heart defect while his brother doesn't. Clearly, said the textbooks, it couldn't possibly be genetic if the twins are identical and they are not both affected." And yet, as a geneticist, Burn noticed that congenital heart defects ran in families; in that respect they certainly behaved like a genetic problem. Moreover, on close examination, the literature on twins revealed a striking excess of heart defects among twins of the same sex, and presumably at least half of them were identical. Was it possible, Burn wondered, that
being
an identical twin caused the defect? "I then spent the next several years collecting lots and lots of twins, both in hospitals and in the community, and the long and short of it is that identical twins are at least twice as likely as singletons to have a heart defect. But typically, like those
 
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boys in the picture, it only happens to one of a pair, which implies that the twinning process itself might have caused the trouble."
Soon after that, Burn encountered the twin with muscular dystrophy. When he looked in the scientific literature, he discovered that differences in X-inactivation had shown up in twin girls before. His colleague Elizabeth Tucker at Queen Elizabeth Medical Center in Birmingham had examined a pair of identical twin girls, one above average in intelligence and the other mentally retarded. Although they were physically indistinguishable during their first year of life, their faces had become increasingly different, so that by the age of ten their parents had decided they must be fraternal. Tucker discovered that the retarded twin suffered from Fragile X syndrome, which is caused by breaks in the chromosomal structure. Burn also found that this wasn't the first time that one of a pair of identical twin girls had exhibited signs of muscular dystrophy and the other had not. Because the disease is carried on the X chromosome, Burn reasoned that the discordance between the two girls must have something to do with X-inactivation. "If you took a normal girl and looked at 1,000 cells, you'd expect 500 of those cells to be using mother's X and 500 to be using father's Xjust like tossing coins," says Burn. "Now and again, very rarely, a girl will toss heads in every cell, and not tails, and switch off all her good copies." That may have been what happened with Jenny. On the other hand, Burn thought, what if the X-inactivation had occurred
before
the zygote split? Could there be a link between the X-inactivation and the twinning event? "I thought maybe what was happening is that every now and again a ball of cells would end up with a preponderance of mum's X on one side and dad's X on the other. That might actually cause
 
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them to separate developmentally. If the two balls of cells are sufficiently different they may say, 'I don't recognize that gang over therethey look different from us. Why don't we make a baby of our own and let them make a baby of their own?' So instead of one girl who is a normal carrier with a mixture of cells, you end up with two girls, one made of cells with the good genes and the other made up of cells with the bad genes." If Burn's theory was correct, then it was the chromosome defect that triggered the twinning, rather than the other way around. Burn thought that if he could understand why Jenny was afflicted and Kate was not, he might find a thread that would unravel the secrets of the twinning process.
As it turned out, Burn was right: the afflicted twin was carrying only her mother's X; her unaffected sister was carrying only her father's X. Could this be characteristic of all identical twin girls? Burn and his colleague Judith Goodship, who was studying X-linked disorders, arranged to collect all the placentas of twin girls born in and around Newcastle over a period of two years. DNA tests of the umbilical cords determined whether the twins were MZ or DZ. Just as Burn had predicted, the survey showed that identical twin girls were more likely to have skewed distribution patterns of the X chromosome. The rest of his theory, however, posited that if one twin had an excess of the mother's X, then the other would mirror her by having an excess of the father's; it was something about that tug-of-war that caused the zygote to break apart in the first place. What he and Goodship found, however, was that if one twin had an excess of mother's X, the other was just as likely to have only father's, or some of mother's and some of father's, or all of mother'sit didn't seem to matter. Therefore the X-inactivation probably took place
after
twinning, not before. However, MZ girls as a group showed more skewing
 
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than DZ girls. "Whatever causes identical twinning," says Burn, "these studies show that even though they share the same genes, a genetic trait does not have to be shared. The English language is misleading in calling these twins identicalperhaps we should use the German term
eineiige
, 'one egg.'
"We still need to understand why monozygous twinning is so common in humans and so exceptionally rare in other species," he continues. "I now think that it has something to do with chronology. If you start messing around with the exact timing of ovulation, and fertilization, and implantation, then you might end up creating the conditions that allow twinning to occur. No single magic bullet will explain everything. Some twins will be genetic, some twins will be a case of disturbed development. There might be a fundamental law of physics that the very things that hold cells together will every now and again just come apart. So maybe we shouldn't try to explain everything at once.
"What is fascinating about twins is that they are a condition of humanity which forces all rules to be qualified. Everybody is differentexcept identical twins. Everybody has got different blood groupsexcept identical twins. And they are so dramatically visible. They are known to everyone, every writer is aware of the dramatic potential of twins, every kid who ever met identical twins is instantly fascinated by them. They have been known since we have been able to know about anything. Yet we still don't know why they happen. We have all sorts of hypotheses that are given as fact. I now regard a fact as a hypothesis that people don't bother to argue about anymore."
Clearly, some identical twins are more identical than others. "Identical twins are less alike than people think
 
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they are," says Nancy L. Segal, a professor of developmental psychology and the director of twin studies at California State University, Fullerton. "Some of the differences are traceable to the early prenatal stagesunequal nutrition, unequal blood supplies. Twins never have completely identical fingerprints. If twins split late there can be differences in mirror-imaging effectsone twin may be right-handed, one left-handed; they may have opposite hair whorls, opposite dental patterns, and opposite birthmarks and moles. While I think identical twins are more alike than any other pair of people you can put together, nevertheless they fall into a relationship vis-à-vis one another, so that one might dominate in certain situations and one might be more submissive. There's a spectrum along which twins differ." Fraternal twins can be so similar that they believe they must be monozygotic, while identicals can be dramatically discordant for facial features, such as cleft lip or palate. An MZ twin who is the product of a placenta that was only marginally attached to the womb may develop into a miniature version of his siblingas in the premise of the film comedy
Twins
, in which Danny DeVito and Arnold Schwarzenegger play a long-lost identical pair. "None of the correlations that we look at are 1.0," says Segal. "There's differences in virtually everything.''
In an examining room at Queen Charlotte's and Chelsea Hospital in London, Dr. Nicholas Fisk placed an ultrasound scanner on the swollen belly of a woman in her nineteenth week of pregnancy. The woman, whom I will call Anita, had been referred to Dr. Fisk because she was carrying twins. The twins were genetically identical, but through some accident inside the womb they had become grotesquely unequal companions. One appeared to be healthy and normal, the other had no head,
 
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and its heart contained only a single chamber rather than the usual four. "Obviously, a baby without a proper head doesn't survive," said Fisk, as the monitor read the staticky signal from the scanner. "It is acting as a parasite and getting all the nutrients from the healthy baby, who is our main concern. It is quite difficult for the healthy baby to pump all that blood around to the other twin, which can lead to heart failure."
Anita was considering an experimental procedure that would block the blood flow into the parasitic twin. "This gives us a chance to have one baby perfectly well and healthy," she said. She turned to look at the ultrasound monitor, where the ghostly image of a four-month fetus swam into view. One could see all the way through to its skeletal structure and the vague outlines of its internal organs.
"Here we have the healthy baby," said Fisk, who began taking measurements of its thigh on the screen. "Do you feel any bigger, Anita?"
"Slightly."
"It strikes me that it hasn't grown a lot. The heart structure looks completely normal, however," said Fisk as he moved the scanner over the baby's head. "Now here's the baby's face. Can you see that, Anita? You can tell this one is quite healthy because it's moving. Sick babies do not move." He slid the scanner to the right side of Anita's belly, and a totally different creature appeared on the screen. It was a formless mass of tissue with no visible skeleton. The umbilical cord inserted directly into the primitive, single-chambered heart. By using color Doppler Fisk watched the blood flow from the healthy twin into its parasitic sibling.
Anita watched it for a moment, then turned away. You might view it as ironic that I'm a twin," she said. "I'm best friends with my twin brother." She had been
 
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hoping for twins. "Maybe we'll still have a chance someday."
Anita's twins suffered from a rare condition called chorioangiopagus parasiticus, or acardia, which is found only in identical twins. It occurs in about one in 30,000 deliveries and usually results in the death of both fetuses. It is an extreme but vivid example of how twins can be genetically identical but biologically different. Most MZ twins share their blood through anastomoses, which are like arteries and veins that run between the twins inside the womb. One sibling usually gains an advantage over the other, with the result that there is typically a greater disparity in size between MZ than DZ twins at birth. Just because twins are "identical" in their genes doesn't mean that they are anatomically the same.
"All of us have got things like moles and skin tags or one finger that's larger than the other," says Judith Hall. "We've all had a mutation in every single gene in our body. Whether it causes a problem depends on what tissue and how early or how late it appears in development. In fact, our bodies are so smart they basically know how to handle mutations and for the most part they just carry on. But if it happens at a very early stage, when cells are growing very rapidly, the self-correcting mechanisms don't seem to be there." Most twins are sufficiently distinct physically that they can easily be distinguished by their parents and close friends. Some identical twins are so different that they don't resemble each other any more than ordinary siblings do.
There have even been at least three instances of genetically identical twins of mixed gender. In each case this confounding event occurred because the female was born with only one X chromosome and no Y (it takes a Y chromosome to make an embryo male). Somehow,
 
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possibly because of the twinning process, one twin was stripped of the Y chromosome, and became a female by defaultalthough a special kind of female. These individuals suffer from what is called Turner's syndrome. They are short, broad-chested, with extra folds of skin around the neck, and rudimentary sexual development. The study of Turner's syndrome, which occurs in one out of 10,000 female births, suggests that a second X chromosome is necessary to create normal ovaries and ordinary female growth.
In the strange world of twin studies there is an especially bizarre, but highly influential, case of identical twin boys, one of whom was raised as a girl. Dr. John Money, the distinguished sexologist at Johns Hopkins University in Baltimore, reported the case in his 1972 book (with Anke A. Ehrhardt)
Man and Woman, Boy and Girl
. In October 1963, when the twins were seven months old, the parents took them to a hospital to be circumcised. The doctor used an electrical cauterizing needle rather than a scalpel to perform the procedure. Unfortunately, the doctor applied so much electrical current to the first twin that his entire penis was burned off. A plastic surgeon suggested to the distraught parents that they might consider "reassigning" the sex of the child, on the basis that a vagina is easier to construct than a penis. The parents resisted, but they happened to see a television program about the work being done at Johns Hopkins with transsexuals. "We gave them advice and counseling on the future prognosis and management of their new daughter," wrote Money. The parents changed the name of their "daughter" and began treating her as they would a little girl. Extensive plastic surgery (including castration) and hormonal treatments followed. The daughter was not told that she was a son.

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