Twins: And What They Tell Us About Who We Are (16 page)

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Authors: Lawrence Wright

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

BOOK: Twins: And What They Tell Us About Who We Are
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single egg, one that splits
before
conception, so that the same egg is fertilized twice. That would confound comparisons between identicals and fraternals, undermining the whole structure of behavior genetics.
There was a way of testing at least part of Boklage's hypothesis. In Belgium, Catherine Derom and her colleagues at the Center for Human Genetics in the Catholic University of Leuven were looking into the relationship between artificially induced ovulation and multiple births. The connection between the two was well known and easily explained. Artificial ovulation made more eggs available to be fertilized. Therefore any increase in twins should be accounted for by the fertilization of the superfluous eggs. In short, all the twins should be fraternal. Derom reasoned that if there were any connection at all between fraternal and identical twins, then there would also be an increase of identical twins among the population of children born after artificially induced ovulation. She examined twins born by artificial induction in East Flanders between 1964 and 1985. To the astonishment of nearly everyone in the field, Derom discovered that identical twins were being born at a rate three times higher than normal. She was at a loss to provide a satisfactory explanation.
"Some of the things we don't know yet are whether all the disappeared twins are monozygous or dizygous," says Judith Hall. "We don't know whether the increased incidence of congenital anomalies that is reported in all twins is primarily in monozygous twins or whether it is actually found in both kinds of twins. We still have a lot of work to do in establishing whether twins come from one egg or two eggs. Now that we have DNA markers, I think we'll get much better data about what are the differences between dizygous and monozygous twins. In general, it's thought that dizy-
 
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gous twins are caused by high levels of hormones that make you ovulate. By contrast, nobody has a clue why monozygous twins happen."
The rate of dizygotic twinning varies wildly among various countries and ethnic groups, ranging from a low of six twin pairs per 1,000 births in Japan to forty-five pairs per 1,000 in Nigeria. The tendency to have fraternal twins appears to be both genetic and environmental. Broadly speaking, Orientals have the lowest rate, Caucasians an intermediate rate, and Africans the highest rate of twinning, but there are remarkable differences within populations that are genetically very much alike. The Yoruba tribe in Nigeria are the world champions of twinning: as many as one out of eleven Yorubans is a twinfar more than other tribes in the regionalthough the rate of identical twinning is no greater than in Europe or the United States. The high rate of fraternal twinning among the Yoruba may be accounted for in part by diet. The tribe eats a species of yam particularly high in estrogen, which causes an increase in follicle-stimulating hormone (FSH), which may cause higher rates of ovulation. When the Yorubans move to the city, the twinning rate declines, presumably because their diet changes, although there is also a correlation between high twinning rates and agricultural labor, for whatever reason.
Twinning has also been shown to be genetically influenced and carried through the maternal line, although in several families the trait appears to have been passed along through the father's genes as well (this point is still hotly disputed, however, like so much of twin research). Tall, heavy women give birth to twins more often than short, thin women, perhaps because they are better nourished. Older women are far more
 
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likely to conceive twins than are younger women, probably due to higher rates of FSH secretion. The twinning rate begins to fall after age thirty-seven, which may be the result of the increased likelihood of spontaneous abortions.
Twins also happen more frequently to unwed mothersforty percent more often than to married mothers, according to one Swedish study. The explanation offered is that irregularity of intercourse allows the awaiting egg to go unfertilized for a longer period of time; as the egg begins to decay in the uterus, it becomes more prone to splitting. Undermining both of these observations are the contrary data showing that twins are conceived more frequently in the first three months of marriage, when sexual activity is higher and the mother is younger. Rates rise and fall over time, so that twins were only half as frequent in Sweden in 1960 (before the widespread practice of in vitro fertilization) as they were two centuries before. Seasonal variation may affect twin conceptions; for instance, more twins are conceived in Finland during the long summer days than during the long winter nights, confounding other intuitive notions about human behavior.
One prodigious Texas woman, Sarah Womack, had a set of quintuplets, a set of quadruplets, three sets of triplets, five sets of twins, and nine singletons, producing a total of thirty-seven living children between 1911 and 1933; but the record is held by a nineteenth-century Russian woman who bore sixty-nine children to a peasant named Feydor Vasilet: sixteen twin pairs, seven triplets, four quadruplets, and no single children at all. (Vasilet, incidentally, married a second time and had eighteen children by his second wife in eight pregnancies, his last child was born in 1872 when Vasilet was seventy-five years old.) Since the turn of the cen-
 
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tury there have been two cases of nonuplets, five octuplets, ten septuplets, and twenty-three sextuplets reported, but few individuals from those births survivea testament to how perilous multiple birth can be. There are three living sets of sextuplets in the United States (ten in the entire world), fifty quintuplets and more than 400 quadruplets. The largest number of
identical
multiples were the Dionne quintuplets, born in Canada in 1935. Another set of quintuplets, born in Argentina in 1943, consists of a pair of identical twin boys and a set of identical triplet girls.
In the United States about eleven births out of 1,000 are DZ twins, meaning that about one person in fifty has a fraternal twin. The ratio of live multiple births to all live births rose 214 per cent between 1980 and 1994. Triplets increased at seven times the single-birth rate. The number of twin births skyrocketed by thirty-three percent in 1994 alone, the last year for which such figures are available. ''The increase is due to two things," says Louis Keith. "One is the aging of the maternal cohort. The number of women over thirty giving birth to children is increasing. These are the women who've gone to school; they're professional womenbankers, lawyers, doctorswho have delayed their first birth. In the United States fully a third of all first births in 1994 went to women over the age of thirty-five. That's only part of the problem. The real increase in twins and triplets in the United States did not occur until 1985, when we learned how to superovulate women. Now doctors are prescribing ovulation-enhancing agents like they were prescribing bubblegum at a children's birthday party."
The same hormones that Yoruba women get from eating yams are used to assist women in ovulating. They are called gonadotrophic hormones, and they affect the
 
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timing and release of the monthly eggs. A woman's ovary typically uses only 400 eggs out of the half-million she has available. Hormone derivatives shake the tree. Surplus eggs are released into the fallopian tubes, leading to twin conceptions in about ten percent of the women who take such popular fertility drugs as clomiphene citrate (sold under the names Clomid or Serophene). Pergonal, another powerful drug that helps the egg mature, causes twins in about twenty percent of the pregnancies induced by this method. In vitro fertilization and other relatively new and expensive forms of reproductive technology require taking eggs from the ovary and mixing them with sperm in a petri dish; when the eggs are fertilized, they are placed in the uterus or fallopian tube. Typically more than one fertilized egg is implanted, so the chances of multiple births are quite high.
Although Keith is himself an identical twin, he believes that the worldwide twinning epidemic is a medical emergency. "It's a disaster not only in the United States but in many Western countries. The reason is that multiples mirror the two major problems of modern obstetrics: pre-term delivery and low birth weight. Compared to singletons, twins are ten times more likely to be born prior to the thirty-third week of pregnancy and ten times more likely to be weighing less than 1,500 grams at birth. It's worse for triplets, it's worse for quadruplets, and it's worse for quintuplets. It contributes enormously to the cost for medical care."
While the number of fraternal twins varies across cultures and ethnic groups and families, the rate of identical twinning was thought to be constantabout 3.5 twin births per 1,000until recently, when the number of identical twins inexplicably began to rise. About one person in every 150 is an identical twin. In the United
 
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States that means that about one-third of all twins are identical, but in Japan, with its low rate of fraternal twinning, nearly two-thirds are identical. This makes statistical estimates between populations rather unreliable. Traditionally, identical twins were distinguished from non-identicals on the basis of sex and appearance: same-sex twins who looked alike were assumed to have been born of the same egg. If the twins were born in the same gestational sac, that was supposed to provide clear proof that they were identical; if they were in separate sacs, they were ruled fraternal, no matter how closely they resembled each other. DNA tests have shown that both of these assumptions are faulty. About a third of identical twins are born from entirely separate placentas, and occasionally the placentas of fraternal twins merge into one. Many same-sex twins who believe that they are fraternal may actually be identical, and vice versa.
 
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7
The Same, but Different
In 1981 an unusual pair of twin girls found their way to the office of John Burn, who was at the Great Ormond Street Hospital for Sick Children in London and is now a professor of genetics at the University of Newcastle-upon-Tyne. Katy and Jenny were friendly, easygoing, eight-year-old girls, who both wore their dark hair in pigtails. Both had distinctive dark, round eyes, slender lips, and long faces that finished in jutting chins. Katy was a gifted gymnast with a lively expression and a bouncy athletic gait. Jenny was notably shorter and weaker. Her calves were swollen, giving a false impression of muscularity; in fact, she had difficulty standing, and she walked with an obvious waddle. Her scores on verbal intelligence tests were also significantly lower than Katy's, reflecting the duller expression in her eyes. Further tests demonstrated what Professor Burn suspected but could scarcely believe: the shorter, weaker, less intelligent twin was suffering from muscular dystrophy; the other twin showed no signs of the disease. And yet according to DNA tests, the twins were identical.
The twins presented, as it were, twin mysteries. Like hemophilia, Fragile X syndrome, and red-green color
 
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blindness, muscular dystrophy is carried on the X chromosome and is consequently known as an X-linked disorder. Specifically, it is a flaw in the dystrophene gene, which gives integrity to the muscle membranes and allows them to contract and relax. The particularly savage form of the disease manifested in Jenny, called Duchenne, is a progressive wasting illness that is almost always fatal by the age of twenty. It is usually thought of as a disease that affects only boys.
Females, of course, carry two X chromosomes, whereas males have an X and a Y. When a female zygote discovers that it has twice as many X chromosomes as it requires, an interesting process follows, called X-inactivation: in each cell half of the X chromosomes are turned off. Nature provides a striking way of visualizing this in the form of calico cats, which are exclusively female. Genes for fur colors are carried on the X chromosome: there is one gene for black-and-white coat colors and another gene for the orange-marmalade color. A male cat may be either black and white, or orange, but the female, through X-inactivation, may choose one or the other X chromosome for each pair, producing the calico's patchy coloring.
X-inactivation usually takes place about sixteen days after conception, after twinning is thought to occur. Usually the inactivation is a completely random process; a girl who carries a flaw on her dystrophene gene should have about half of her muscle cells in good working order. Female carriers who do show signs of Duchenne have milder symptoms, and the disease rarely advances. This is because females have the opportunity to select a good gene from their extra X, whereas males are stuck with what their single X offers them. Since few males with muscular dystrophy live long enough to reproduce, the flaw is carried through

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