When Karen and Tom received their new baby, whom they named Kathy, they and their traveling companions noticed the striking resemblance of Kathy and Sarah, Susan and Steve’s daughter. Both babies had brown hair, a round face, round eyes, and tiny chins, and they were extremely petite, actually “waif-like,” with estimated ages that were only seven days apart. Both families suspected that they had received half of an identical twin pair. Susan contacted me, and we obtained saliva samples to compare the girls’ DNA. As I noted earlier, the most accurate way to determine twin type is by comparing twins’ resemblance across about fifteen DNA markers, the small segments of DNA that vary enormously from person to person. The only two people to match completely across all fifteen are identical twins.3
Everyone was shocked when the two little girls did not match on any of the DNA markers, so I persuaded the lab to repeat the test. When they did, the samples matched. It seemed that one of the samples had been contaminated, but which test could we trust? The lab agreed to perform the test again with a fresh set of samples, but this time the results showed that the girls were not related. So, just to be sure, a different laboratory repeated the test a fourth time and confirmed that the girls were not related. Sarah’s mother, Susan, was especially disappointed, asking sadly and hopefully whether they might be cousins.
* * *
Because of the profound implications of the DNA test results for all four men and their families, we had to address the real, but unlikely, possibility that the Colombian twins were chance look-alikes. Analysis of their DNA confirmed what everyone could see, that Jorge and William, and Carlos and Wilber, were the real twins, and both pairs were identical.4 When I explained the actual procedure to Carlos over drinks, he joked, “Just don’t give me another clone!” In fact, identical twins are clones by definition—genetically identical organisms—but human clones would not be identical twins because they do not meet the unique criteria that define twinship, such as simultaneous conception, common intrauterine environment, and shared date of birth.5
Dr. Juan J. Yunis, a medical geneticist, has worked with several Colombian families after single babies born to different mothers were inadvertently switched. “We see many things in our laboratory, so I am not surprised by much,” he acknowledged, but nevertheless he called the Colombian twins’ situation “a spectacular case.” The father of Juan J. Yunis, Dr. Emilio J. Yunis, was the geneticist who performed the analyses of the Colombian twins’ DNA.
Chorions, Amnions, and Placentas
One of the great myths surrounding twins is that fraternal twins always have two placentae, whereas identical twins share one. The placenta enables the transmission of oxygen and nutrition from mother to fetus, as well as the release of waste materials and carbon dioxide from the fetus. The truth is that some fraternal twin pairs have fused placentae, giving the appearance of one, and about one-third of identical twin pairs have two, such that each twin has his or her own. A related myth is that all fraternal twins arrive individually encased in their own chorion and amnion, the outer and inner protective membranes surrounding the fertilized egg, whereas identical twins always share these structures. In fact, some unusual fraternal twins—and we don’t know how many—and about two-thirds of identical twins have a single chorion, although their amnions are separate. Only a tiny percentage of identical twins, perhaps 1 to 2 percent, share their chorion and their amnion, and would be at high risk for transfusion syndrome, or TTTS.6
Birth Records and Birth Weights: Telltale Signs?
The birth records for Carlos and Wilber, born in the small country hospital in Vélez, could not be found. However, Carlos was probably delivered first because the twins’ mother, Ana, claimed that William was born first but, of course, William and Carlos had been switched.7
The complete chart for the identical twins William and Jorge, born at the Hospital Materno Infantil in Bogotá, was available and shows that William was delivered first, followed by his brother seven minutes later. The twins were classified as “monochorionic-diamniotic” (one chorion, two amnions), making them members of the identical twin majority at some risk for TTTS, but they exhibited no symptoms of this condition. The only possible sign was that William was anemic, but he was the larger twin, and the smaller twin typically shows anemia if TTTS has occurred. This made me wonder whether their birth orders written on their charts had somehow been switched, a possibility that could be a telltale sign that one baby had been replaced by another, which fits with something I learned a little later. However, their birth weight difference was less than the 18 percent threshold that currently identifies twins at risk for TTTS or other health concerns, so physicians don’t usually worry about twins’ condition unless their birth weight difference reaches this threshold. This difference is lower than that indicated in the past.8
The Bogotá twins’ birth weights were slightly below average for male twins delivered at thirty-five weeks. William weighed 5 pounds, 1 ounce, while Jorge weighed 4 pounds, 8 ounces. However, both twins’ Apgar scores (values reflecting a newborn’s physical condition on a 1 to 10 scale) of 7 and 10, presumably taken at one and five minutes after their birth, show that their physical health was good, so the twins probably went to the premature nursery for precautionary reasons.9 But William and Jorge’s medical charts contain something strange that was apparently overlooked.
A reasonable assumption is that the larger twin at birth was the larger twin at discharge. If that was so, William gained nearly a pound, whereas Jorge lost just about one-tenth of an ounce. Babies often lose 5 percent of their weight in the first few days, start to gain weight by days 3 and 4, and end up back at their birth weight by day 5. So far, nothing was amiss.10 But the records do not make clear which discharge weight goes with which twin. Reversing the discharge weights means that William would have lost about nine ounces, while Jorge would have gained a little more than 1 pound, 5 ounces. Newborns can be safely released from the hospital even if they have not fully recovered their weight loss, but a more prudent plan is to wait until babies put on some pounds.
What could have happened? Because we know that William and Carlos were switched, a possible—and, I believe, likely—scenario is that the medical chart contains William’s birth weight of 5 pounds, 1 ounce, and Carlos’s discharge weight of 4 pounds, 5 ounces. Ana, Carlos’s biological mother, said that Carlos weighed about two pounds at birth. The discharge weight of 4 pounds, 5 ounces, is more consistent with Carlos’s low birth weight than it is with William’s. However, it is unlikely that Carlos would have nearly doubled his weight in four or five days, so he may have weighed more than two pounds at birth; Ana was in surgery and may not have recalled this information correctly. Regardless, if someone had carefully checked these numbers, then William’s possibly erroneous weight loss of 9 ounces—and the exchange of babies—might have been detected almost immediately.
It seems extraordinary that a baby delivered at thirty-five weeks and weighing more than five pounds was switched with a baby delivered at twenty-eight weeks and weighing about two pounds, especially a newborn who needed immediate medical care. Seven different nurses made notes in the medical chart on December 22 and 23, suggesting that no one in the hospital really got to know Luz Marina and her babies. And, interestingly, in places the medical chart reads as though it concerned only one baby.
Growing Up
All four premature twins thrived physically with the love and care they received. Seeing them today as grown men belies the fact that they had once been small and fragile. Their body builds follow their genetic lines—Jorge and William are shorter and slighter than Carlos and Wilber. However, William is physically the strongest, a legacy of his years of hard work on his father’s farm; he surpasses the other three in endurance, stamina, and arm wrestling. He showed impressive speed in beating his twin in a good-natured arm wrestling joust. Neither competition nor ill will was evident in this interaction—it was just twins at ease being together and having fun.
Two p
eople may weigh the same, but a tall person might appear to be too thin and a short person might look too fat. Body mass index (BMI), calculated as weight in kilograms divided by height in centimeters squared, is a convenient way to know whether your weight is too low, too high, or just about right for your height. Genetic influence on BMI is about 85 percent, evident in the greater similarity of identical than fraternal twins.11
Identical twins are the privileged few who can really know how they look sporting a new outfit, a different hair style, or an extra twenty pounds since they have only to look at their twin. One of the reared-apart female twins in the Minnesota study always thought she was too fat, but when she met her twin sister, who was ten pounds heavier and quite attractive, she changed her view of herself. And a reared-apart male twin showed up at the study weighing eighty pounds less than his identical twin brother. Seeing his twin made him grateful that he had stuck to his diet.
The BMIs of the four twins were similar in 2015, seven months after they met, because the shorter twins weighed less than the taller twins. But during the next year William gained nearly twenty-one pounds by pumping iron and guzzling high-protein cocktails, which reset his BMI to just short of overweight. Here we have compelling evidence of how a lifestyle change can reduce identical twins’ physical resemblance. With his broader face and larger torso, William did not resemble his twin as much as he did the year before. What if he had started working out a year earlier? Jorge’s coworker Laura might not have confused them, they might never have met, and we would not have written this book.
Despite his weight gain, William’s new BMI was still closer to his twin’s than to those of the unrelated brothers with whom he was raised in La Paz. Carlos’s BMI was closest to Wilber’s, reflecting the effects of their genes, although Wilber’s BMI was slightly closer to that of his biological brother Chelmo.
William likes his new body because women take greater notice than they did of his skinnier self. His twin brother, Jorge, is also pleased with his own appearance, but hopes to reduce his “big belly,” hardly noticeable to others (and maybe not even to Jorge until William buffed up). Jorge walks and rides a bike, but he won’t exercise like his twin, whom he criticizes as being obsessed with fitness.
* * *
Identical twins often monitor one another, tracking the physical changes that make them more or less alike. Carlos likes his appearance, but growing up he lacked self-confidence because he thought he was too skinny. Adding weight took care of those feelings, but now he is at an impasse—he wants to lose fat but not weight, although he knows that losing weight will help him lose fat. He also would like to have larger legs, but that would be possible only by gaining weight or working out, neither of which he finds an attractive option. When it comes to legs, when Carlos and Wilber walk side by side, their calves show a similar outward curvature, an expression of their identical genes.
“Carlos is fatter now,” Wilber observed in 2016, aware that his twin could also see that Wilber’s stomach had grown. Wilber eventually flattened his middle by forfeiting afternoon naps and evening meals. Then he lost too much weight, resumed eating his favorite foods (catfish and pork ribs), and regained several pounds. Overall, Wilber likes his looks, and both twins call special attention to their deep reddish lips, teasing their girlfriends about how deliciosos they are. The field known as cheiloscopy studies lip prints, which are made by pressing the lips against a special surface. They are actually like fingerprints because the patterns of lines and creases are unique to each individual. And the lip prints of identical twins are alike, but not exactly alike.12
The high-end clothing choices of Wilber and Carlos—which between them include a Diesel watch, Zara shirt, Gucci bag, and Kenzo jeans—reflect their focus on style and appearance. These two always look like they have just been to a hair stylist. Carlos sometimes tells his twin to stop imitating him, but Wilber is just doing what comes naturally. Their preferences encourage us to think about why we dress as we do. Choice of clothing arises from a combination of genetic and environmental factors, including body type, fabric texture, item availability, fashion trend, and purchasing power. Manner of dress also partly reflects behavioral attributes, such as self-monitoring (observing and managing one’s own behavior—high self-monitors tend to use attire to manage the impressions they make on others) and self-perception (one’s sense of value and self-worth—those who believe they are competent and dependable at work generally dress to impress). Genetic factors partly influence both self-monitoring and self-perception.13
Identical twins raised in different environments probably end up alike in these surprising ways because of active gene-environment correlation, the purposeful seeking out of opportunities and events compatible with genetically influenced interests, personalities, and temperaments. We all do this, creating our own environments from what is available to us. In La Paz, Wilber wasn’t privy to the fashion trends and clothing choices his twin brother, Carlos, enjoyed in Bogotá, but Wilber made the most of what he could see and learn. Once he moved to Bogotá, the many choices and trends around him amplified his fashion interests, resulting in a good gene-environment fit.
Hands and Fingers
Twins are a curious group when it comes to which hand they favor. About 20 percent of twins are left-handed, about twice the percentage of nontwin lefties in the general population. A once-plausible but now contested theory is that late splitting of the fertilized egg, at or after day 8 postconception, is unlikely to yield precise copies of the original, perhaps causing left-handedness in one twin. Twins are, however, more likely than nontwins to experience birth hazards, such as difficult delivery and low birth weight, which have been linked to left-handedness in some pairs. Parents of twins are more often left-handed than their siblings who are parents of nontwins, although none of the parents of the Colombian twins wrote with their left hand.14
About 25 percent of identical twins are opposite-handed, meaning that one twin is right-handed and one twin is left-handed. This same minority of identical twins variously shows mirror-image eye dominance, fingerprint patterns, dental characteristics, and/or hair whorl, the direction in which the hair spins at the top of the head. Unusual biological events affecting laterality (sidedness) as the fertilized egg develops might cause identical twins to mirror one another in these ways. Environmental influences after birth, such as differences in smoking habits, tooth extractions, and denture wearing, have also been tied to facial reversals between twins. For example, when one twin smokes and the other does not, there can be differences in the deviation of the nasal septum, the wall separating the two nostrils.15
* * *
All four twins are right-handed, but not to the same degree. William is the strongest right-hander, followed by Wilber, Jorge, and Carlos in that order, with Carlos on the brink of ambidexterity, followed closely by Jorge. Both Carlos and Wilber are among the minority of “inverted right-handers,” pointing their pencil downward and holding their hand above the line of writing. Santi, Jorge’s young son, does this too. The little boy is not biologically related to the identical pair Carlos and Wilber, but links between twinning and left-handed tendencies have shown up in some families, and Santi’s father, who is not a strong right-hander, is a twin. Perhaps when Carlos and Wilber, as well as Santi, were younger, their parents, caretakers, or school system discouraged their attempts at using their left hand for writing or drawing. Forcing them to use their right hand instead of their left may have affected their hand posture for reasons still not understood. Interestingly, the percentage of left-handers in Latin American countries is smaller than the 10 to 12 percent found in the Western world.16
Fingerprints
One of the great myths surrounding identical twins is that they have identical fingerprints, leading to the assumption that if one twin commits a crime, the other twin could be falsely accused. But that just isn’t so. Identical twins do not have identical fingerprints because of variations in the womb such as temperature and
positioning. However, their fingerprints are more alike than those of fraternal twins, indicating some genetic influence, especially for patterns known as arches.17
The science of fingerprints is called dermatoglyphics. Fingerprint ridges are the raised lines found on the tips of our fingers and are formed continuously between the tenth and twenty-fifth week of gestation.18 Analyses may consider the total ridge count (sum of the ridges crossing the ten fingers) and pattern types (arch, loop, whorl, and variations of these three main types present on each finger).
As expected, the smallest differences in total ridge count are between the real identical twins, Jorge and William, who differ by twelve, and Carlos and Wilber, who differ by seventeen. And the greatest differences are between the unrelated brothers who grew up together, Jorge and Carlos in Bogotá (thirty-one), and William and Wilber in La Paz (sixty). Genetic effects are clearly at work, although neither identical pair matches perfectly (identical twins rarely do). This raises the question: What is the likelihood that they would be classified as identical based on their ridge count alone?
Kevin Haroian, my former colleague at the University of Minnesota, computed a number called Slater’s Z score that could give us a hint. These scores say that the probability that each Colombian twin pair was identical was more than 50 percent (that is, greater than chance) and that the unrelated pairs were unlikely to be classified as identical. Still, reputable researchers would never rely solely on fingerprint ridge count for determining twin type, because identical twins could have markedly different fingerprints if one were especially stressed while in the womb.19
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