Twins: And What They Tell Us About Who We Are (14 page)

Read Twins: And What They Tell Us About Who We Are Online

Authors: Lawrence Wright

Tags: #Family & Relationships, #Siblings, #Science, #Life Sciences, #Genetics & Genomics, #test

BOOK: Twins: And What They Tell Us About Who We Are
11.4Mb size Format: txt, pdf, ePub
 
Page 82
C. Pillard, a psychiatrist at the Family Studies Laboratory of Boston University School of Medicine, studied 161 gay men, each of whom had an identical or fraternal twin or an adopted brother. They found fifty-two percent of the identical twins of gay men were also gay, compared with twenty-two percent of the fraternal twins and eleven percent of the adopted brothers. Bailey did a separate study of Australian twins, however, and found that the chances of a gay twin having a gay brother were only twenty percent among identicals and nearly zero among fraternals. In Bouchard's reared-apart twin sample there are two sets of twins with homosexual men; in one set both twins are gay, in the other only one is. A larger sample of more than ten thousand twins found that the heritability for sexual orientation in women was nearly fifty percent, but in men it was zero.
Even if homosexuality in men is shown to be, for instance, thirty percent heritable, what, exactly, is inherited? "The fair thing to say is that nobody knows," says Pillard. "Is it a propensity to like somebody who is sort of the same as you versus somebody who's different, or to like a man versus a woman, or to be very sensitive or what? To me that's the payoff question. What is it doing up there in that little brain that's making you different? What's the mechanism for that? It's hard even to speculate because we don't know that much about how genes affect behavior."
One reason is that research has been discouraged. When Dutch scientists announced that they had found a gene linked to aggression, they were denounced for even considering violent behavior as a genetic expression. The U.S. National Institutes of Health canceled a planned 1992 conference at the University of Maryland, "Genetic Factors in Crime," and scaled back its research on causes of violence. (The conference finally
 
Page 83
took place three years later.) Again and again, when genetic research turns toward human nature, rather than simple biology, politics swamps the discussion and often sinks the research efforts. In most instances, the only proof of genetic influence on personality and behavior still comes from twin and adoption studies. As a result, there is a magical air about them, because they traffic in taboo ideas, demonstrating effects without revealing causes.
 
Page 85
6
Twin Mysteries
In 1973 Jerry L. Hall, of the George Washington University Medical Center, reported that he had succeeded in cloning human embryos, using a method that had already been perfected with animals. Dr. Hall merely split the embryo, then at the stage of only a few cells, into two or three parts. Each lump of cells developed normally into twin or triplet embryos. The object of the experiment was to make more embryos available for women who were undergoing in vitro fertilization. Those embryos that were not implanted in the womb were discarded, but they could easily have been frozen and used later, perhaps in other women. Thus the study of twins gains a weird pertinence for the future, as we envision twins separated not only by distance but by time as well.
Whether the clones are formed by splitting embryos or by some other technique, like the one used by Dr. Ian Wilmut at the Roslin Institute in Scotland to clone an adult sheep, they will share the same genesjust like identical twinsand similar problems of identity, but they will create new puzzles of kinship. For instance, an adult female who clones herself creates not only her child but her twin sister. The child's genetic parents are the people she would otherwise identify as her grandparents.
 
Page 86
Clones may in some respects be more identical to each other than naturally occurring identical twins. Twin fetuses can do a lot of harm to each other, and clones grown separately would not have to endure the competition in the womb. Also, there may be something about the splitting process that results in twins that may also cause birth defects. Mice embryos that have been artificially divided and allowed to develop tend to be weaker than normally developed mice. It is possiblein fact, likelythat clones could be created without the same liabilities that attend the twinning process.
During the broad public debate about the morality of cloning human beings, clones have repeatedly been described as nothing more than identical twins, as if that robbed the debate of its mystery. The truth is that the twinning process is much less well understood than cloning, which is a rather simple procedure. In spite of the burst of twin-based scholarship in recent years, much that is commonly believed to be true about twinning is either wrong or in dispute. It is not clear, for instance, whether twinning is a kind of birth defect or, contrarily, whether birth defects are caused by twinning (or if, indeed, either has anything to do with the other). We don't know what significance, if any, to attach to the elevated incidence of left-handedness among both kinds of twins. It is not even certain whether fraternal twins always come from two eggs or sometimes from one that has split before fertilization, creating a third kind of twin. We are only now learning that twins are different in particular ways from singletons (their teeth are less symmetrical, for example), but we don't know why or what that means. In sum, we don't know who twins are or how twins happen. We only presume to know what they tell us about who
we
are.
 
Page 87
So much depends on a phenomenon about which we know so little. Even the prevalence of twins is a subject of puzzlement and controversy. With the increasing use of ultrasound to detect early pregnancies, we now know that twinning is a far more common occurrence than anyone had previously imagined. Although only about one out of eighty or ninety live births produces twins, at least one-eighth of all natural pregnancies begin as twins. Many of us singletons, in other words, began life as something moreas part of a pair.
The use of sonography to detect pregnancy dates from 1957, but it didn't become widespread until the late seventies. By that time, we already knew more about the ocean floor and the dark side of the moon than we did about embryogenesis. Before ultrasound, it was imposible to diagnose accurately twin pregnancies, and most multiple births came as a surprise. As ultrasound became more common and increasingly more sophisticated, however, doctors began having the unnerving experience of viewing twin embryos one month, only to find a singleton the next time they looked. What was happening? In 1980, at the Third International Congress on Twin Studies in Jerusalem, this question was raised, and one of the participants cried out, "Vanishing twins!" thus giving a name to a phenomenon that has caused as much confusion as excitement.
Usually the only external sign of a vanished twin is vaginal bleeding. More advanced ultrasound equipment, including vaginal ultrasound, high-speed scanners, and color Doppler (the same kind of radar apparatus meteorologists use to track the movement of weather), has been able to detect multiple pregnancies as early as five weeks after conception. A 1992 Israeli study, using transvaginal sonography, diagnosed eighty-eight multiple gestations among women who had undergone ovulation
 
Page 88
induction. Of the fifty-four twin pregnancies detected in this group between five and six weeks, fifty-one resulted in the birth of singletons and three were spontaneously aborted; the rest had vanished. There were also twenty-six triplet gestations, producing twelve pairs of twins, twelve singletons, and two complete miscarriages; five quadruplet pregnancies, which produced three triplets, two twins, and a singleton; and three quintuplet pregnancies, two of which were deliberately reduced by doctors to a set of triplets and a set of twins, but there were three vanished fetuses as well. Two different studies found that the frequency of twins among abortions is three times higher than the frequency of twins at birth.
"People are picking up twin pregnancies the size of garden peas. They're seeing a lot more twins than they ever knew were there," according to Charles E. Boklage, a geneticist at East Carolina University School of Medicine and a well-known maverick in the world of twin biology. Boklage has a knack for uncovering paradoxical data that undermine many of the commonplace assumptions people still harbor about twins. "The so-called phenomenon of the 'vanishing twin syndrome' is neither phenomenal nor a syndrome," Boklage contends. "It is much too common to be considered phenomenal, and it occurs for too many reasons to be considered any kind of syndrome." He says that most pregnancies, whether multiple or singleton, fail in any case, so it is not as surprising as it seems that twins often disappear. "Somewhere in the vicinity of twelve to fifteen percent of usand that's a
minimum
estimateare walking around thinking we're singletons when in fact we're only the big half," says Boklage. He estimates that for every set of twins born alive, there are at least six singletons born who are sole survivors of twin conceptions. ''Now, the implications of this are profound. A huge fraction of the
 
Page 89
population are products of twin pregnancies, and therefore are presumably associated with all the things that known twin pregnancies are associated with in terms of malformations and extra risks."
*
Twins are far more susceptible to the birth defects, spontaneous mutations, and vascular problems that threaten early life. Simply being a twin is stressful and raises the odds against survival because of the competition for space and nutrients. If an embryo disappears in the first trimester, it has probably been absorbed by the placenta or by the other twin, with little or no evidence that it ever existed, except for the tiny image it may once have left on ultrasound. A careful examination of the placenta after birth will sometimes reveal a nodule that turns out to be the remains of a vanished twin.
The death of one twin poses a real threat to the other. Identical twins share the same circulation in the womb, and the death of one could cause a blood clot to pass into the survivor, causing heart damage or developmental problems of exactly the sort twins are famous for. Cysts, called teratomas, composed of bits of hair and teeth and fetal bones, are sometimes discovered in adults. Either they are errant cells that have reverted to a primitive embryological state or they are the remnant of a vanished identical twin. Dead fetuses have also been found inside living children. There is a well-known case in 1949 of such a "fetus-in-fetu," in which five fetuses were removed from the brain of an infant girl in Philadelphia; several less spectacular cases have been reported since, including a six-pound fetus found during the autopsy of an elderly man. These events are probably accidents of timing during early pregnancy. Identical
*
Of course, many of the problems of multiple births are associated with late stages of pregnancycrowding and premature birth, for instanceand they would not be a feature of vanished-twin syndrome.
 
Page 90
twinning is thought to occur on or before the fourteenth day after conception. If the division happens early in that cycle, the embryos will be in separate placentas, like nearly all fraternal twins. By the end of the fourth day, the chorion, which is the outer placental membrane, will have formed, and if the zygote divides after that time, as is the case with two-thirds of MZ twins, they develop in a single placenta. If the division occurs between the fifth and the eighth days, the twins will still be encased in separate amniotic sacs, but if they divide after the eighth day there will be nothing between them. Half of these late-splitting twins die, often strangling in each other's umbilical cords. It is thought that by the twelfth day the division is likely to be incomplete, resulting conjoined or Siamese twins, which occur in about one out of 400 MZ births,
*
fetus-in-fetu, and teratomas. These are, however, only theories. One can also make the case that the twinning process got stuck at the beginning and never advanced. These abnormalities are far more prevalent in girls than in boys, since male twins (like all boys) are more likely to miscarry.
Because all twins compete in the womb for space and nutrition, the experiences of twins before birth are considerably different from those of singletons. "Twins also compete physically," says Louis Keith, the head of the Center for Study of Multiple Birth in Chicago and the editor of
Multipregnancy: Epidemiology, Gestation, and Perinatal Outcome
. One doctor on his staff at Northwestern University Medical School observed twin fetuses fighting. "One punched the other and the other looked startled," says Keith. ''On another occasion, when he was looking with the ultrasound at triplets,
*
The estimates for the overall frequency of conjoined twin births vary significantly, from one out of every 33,000 births to one out of 175,000.

Other books

The Lady Series by Denise Domning
His Uncle's Favorite by Lilian, Lory
This Can't be Life by Cannon, Shakara
Eglantine by Catherine Jinks
The Glass Shoe by Kay Hooper
Poisoned Pawn by Jaleta Clegg