Twinning and Twinship

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Biology 103
2000 Third Web Report
On Serendip

Twinning and Twinship

Allison Hayes-Conroy

Being a twin is an experience most twins view as the most normal circumstance in the world and an experience many non-twins wonder about. Many twins throughout history have been recorded in myth, painting, and photography. In literature, twins have often been used to portray opposites, especially good versus evil, but this is rarely the case. The reality of twinship is something that has been widely studied. What makes twins biologically and psychologically alike and what are the causes of any differences in a pair of twins? What makes a pair of twins identical or fraternal? And what are the causes behind twinning and other multiple births? Twinship and twinning is a complicated, exciting, and sometimes mysterious phenomenon.

In explaining the biology behind twinning, it is important to point out the different types of twins that exist. Fraternal or dizygotic twins occur when a woman releases two eggs, instead of one, during ovulation, making both eggs available for fertilization. Fraternal twins, of course, share a common prenatal environment and normally the same birth date. Since fraternal twins come from both two different eggs and two different spermatozoa, the pair can be either female/female, male/male, or female/male. Identical or monozygotic twins on the other hand, must either be male/male or female/female. Identical twins occur when the fertilized egg, or morula, splits. Because identical twins come from both the same egg and sperm, they are exactly alike in their genetic makeup (3). There is speculation that a third type of twin exists, an "identical/fraternal." These twins are said to be a result of a split in the mother's egg after ovulation but prior to fertilization. In these cases, the twins share identical genes from their mother, but differ in genetical makeup because they came from different sperm and thus have differing genes from their father (1).

Today, testing for monozygoticity in twins can be easily accomplished with DNA testing (3). However, studies done earlier in the century help to explain the traits that identical twins, in effect natural clones, have in common. In some of the classic studies of twins, including one done by Dr.Horatio H. Newman, the following criteria were used in determining monozygoticity. In order to be diagnosed as monozygotic twins, the pair had to exhibit a striking similarity in appearance, so much so that the twins are or have often been mistaken for each other (5). Height, fingertip pattern count, and fingertip ridge count had to be similar in monzygotic twins (4). The pair had to be essentially identical in hair color, hair texture, and hair form, as well as eye color, eye pigment, and iris pattern. Their skin needed to be of the same complexion (unless one was changed by tanning) and their skin had to display the same amount of body hair distribution on the face, neck, and hands. More importantly, the pair had to have virtually the same facial features, types of teeth, irregularities in dentition, as well as similarities in the size and shape of the hands and fingers. With most of the identical twins in the test, the details in their palm and finger patterns showed that one hand of an individual in a monozygotic pair, was more similar to one of the other twin's hands than to the individual's own opposite hand. In other words, the pair exhibited stronger cross resemblance than internal resemblance in their finger and palm patterns (5).

There exists a peculiar phenomenon among identical twins commonly referred to as reversed asymmetry or mirror imaging. If division of the morula occurs between the tenth and thirteenth days of development, the twins are commonly found to be mirror images of each other (1). The twins usually demonstrate this reversed asymmetry in handedness (right or left handed), hair whorl patterns, dentition, palm and fingertip details. While one twin has hair whorls growing clockwise, the other might have counterclockwise hair whorls. In some cases, such twins have birthmarks one the same part of the body, the one on the left and the other on the right (5). This phenomenon holds true not only for identical twins, but all identical multiple births. There can be triplets, quadruplets, even quintuplets who appear to be mirror images of each other, as in the case of the Dionne quintuplets. (These five girls are the only known case of identical quintuplets. Other multiple births can be combinations of monozygotes and dizygotes, the most common being all multiple zygotes fertilized simultaneously) (7). Two of the five identical Dionne girls appeared to be the last to separate from the singular zygote, hence probably between the tenth and thirteenth days, because they, among all of the girls, have the highest presence of reversed asymmetry.

After the thirteenth day of fertilization, twins who begin to split often do not make the full division. The result is what is commonly known as Siamese or conjoined twins. The term "Siamese" twins comes from a famous pair of twin from Siam who were joined at the breastbone (1). Of course, all Siamese twins are monozygotic and, in fact, the existence of Siamese twins helped to prove the existence of monozygotic twins (5). Siamese twins may be joined physically at any part of the body including entire torsos, the top or side of the cranium, hips, and chests. Some cases include sharing of vital organs to the point that the two cannot be separated without harm or even death of one of the twins. In some extreme cases of conjoined twins, one twin is actually within the other in the form of a cyst (7). These cysts form tumors in the abdomen, liver, or brain and contain parts or sometimes all of another fetus. In three documented cases, infants were found to be carrying unborn siblings inside of them. Though Siamese twins are monozygotic, often there exists large dissimilarities between the two individuals in weight, height, and head shape and size. Why these differences occur in all twins is a question that has been studied thoroughly. In the case of Siamese twins, most of the differences would likely be due to conditions in their prenatal environment (5). In all twins, however, reasons for the variation of physical and psychological traits include genetics, prenatal environments, and post-natal environments.

No individual develops without both a basis of heredity and influential environmental factors. Though factors such as eye and hair color are considered purely based on genetics, other factors including intelligence, body weight, and muscularity are markedly influenced by both genetics and environmental factors. Fraternal twins are as separate in genetic origin as any siblings coming from the same parent and thus can be as similar or different as any brothers or sisters except for the fact that their similarity in age often leads them to appear more genetically similar (5). Since identical twins are genetically alike, the reasons for differences between them lie not in the realm of heredity, but in the varying environmental conditions each are exposed to.

In order to explain how factors in the prenatal environment can effect twins and other multiple births, the typical prenatal environment must first be examined. Different sex, dizygotic twins are almost always dichorionic--surrounded by separate chorionic membranes. However, not all dizygotic twins are dichorionic and, similarly, not all monozygotic twins are monochorionic. Also, dichorionic twins can either share a common placenta, or have separate placentas. Thus to be identical twins does not necessarily mean having to share a common placenta nor does it connote being enveloped in a shared chorion, although this is often the case (5).

The prenatal environment of identical twins, often leads to dissimilarities between the twins which can continue into adulthood. For twins who share a common placenta the differences in blood supply, and thus nutrients, to opposite ends of the placenta are sometimes a factor (7). Usually, however, the case with identical twins is that the vascular invasions of the placenta meet toward the center of the placenta and compete for this area. As a result of this competition in a small area, sometimes anatosomes--extensive fusions--of capillaries will appear (4). More rarely the fusions are made vein to vein or artery to artery. This can cause a serious imbalance in blood exchange which in turn produces prominent differences in surviving twins including variations in weight and size. These variations "frequently persist for life" and, beyond size and weight, the differences in blood flow can effect an individual's "vigor and general health" and may even effect one's intelligence quotient (5).

A specific disorder, called Twin to Twin Transfusion Syndrome, is related to this idea of disproportionate blood flow. Twin to Twin Transfusion Syndrome is a disease of the placenta affecting only identical twins who are monochorionic-diamniotic or monochorionic-monoamniotic. One of the twins, the recipient twin, receives too much blood and becomes larger in size. The extra blood that this fetus receives is very viscous and thus the baby's heart over exerts itself in trying to pump blood. When left untreated, eighty to one hundred percent of the recipient babies die of heart failure. The other twin, the donor twin, in contrast receives too little blood. This fetus has very little fluid in its amniotic sac and it is normally smaller. Like recipient twins, eighty to one hundred percent of donor twins will die from heart failure (caused this time by severe anemia) if nothing is done to help. For those who do survive, the recipient twin, though larger, usually fares worse than the donor twin because it is more tired (9).

In an earlier study of both fraternal and identical twins, the average pair differences in weight and height, among other characteristics, were determined. The average weight difference of identical twins of all ages was found to be 4.1 pounds, with one pair reaching a difference as high as the fifteen to eighteen pound range. In the case of the fraternal twins, the average weight difference was 10.0 pounds, just slightly lower than the average for all siblings, with one pair reaching a difference in the thirty three to thirty six pound range. The differences in weight can be attributed to both prenatal and postnatal environmental conditions in both types of twins as well as genetics in the fraternal pairs. The average height difference of the identical twins showed to be a bit smaller than the difference in fraternal twins. The identical twin group showed an average difference of 1.7 centimeters, where the fraternals averaged 4.4 centimeters different. Since significant height differences only occurred in fraternal twins, height is thought to be largely an effect of genetics (5).

Because identical twins are genetically exactly the same, these twins have special appeal to scientists searching for the cause of occurrences such as disease, dissimilarities in intelligence, and other physical and psychological characteristics. Both identical twins and fraternal twins have been used in various studies to determine whether the causes of certain characteristics are genetic or environmental. This so-called nature-nurture question still is covered in much mystery, but generalizations can be made on a number of traits (6).

Differences in physical characteristics such as height, size of head, hand size, and muscularity between twins proved to be small and thus these characteristics can all be said to be least effected by environment and most by genetics. The fact that weight and Binet IQ and Stanford Achievement Test intelligence scores differed in identical twins more than the above physical characteristics shows that intelligence is affected to a greater extent by environment. The largest differences between identical twins occurred in the areas of educational achievement and temperament. This finding shows that educational achievement and personality are even more effected by environment (5).

However, whether it be from similar brain wave patterns, a common growing environment, or some unknown gene, identical twins, reared together or apart, seem to have some striking similarities beyond the obvious of an uncanny resemblance (6). In one study done, identical twins appeared to be concordant, or phenotypically similar in a number of mixed categories. Regarding disease, the three most common incidents of concordance were measles, diabetes mellitus, and rickets showing ninety-five, eighty-four, and eighty-eight percents respectively. Also, according to the study, eighty-nine percent of identical twins with Down's syndrome, will share that disorder with their twin brother or sister. Regarding psychology and psychological disorders, the four most common disorders demonstrating concordance were feeblemindedness, schizophrenia, manic-depressive psychosis, and criminality showing ninety-four, eighty, seventy-seven, and sixty-eight percents respectively (6). Numerous studies show that the concordance rates for identical twins having schizophrenia are always three to four times higher than the rates in fraternal twins (2).. Also, interestingly, Smoking habits, coffee drinking, and alcohol drinking showed high levels of concordance with, respectively, ninety-nine, ninety-four, and 100 percents. All of the above disorders revealed to have lower concordance percentages (much lower in some cases) in the tests done on dizygotic twins (6).

Findings such as these are helpful in explaining some incredible findings about twins reared apart. Two twins, one from California and one from Germany who had been parted at birth both exhibited strange traits such as wearing rubber bands around the wrist, clipping one's moustache in a peculiar way, and sneezing loudly to get attention as well as more common characteristics such as temperament and tempo. Other separated twins shared common phobias and defects originally thought to come from one's growth environment. These results suggest that more psychological traits like leadership ability, imagination, and vulnerability to both stress and alienation may be markedly hereditary (7).

Besides the biologically colorful "human-interest" stories of strikingly similar separated twins, there are many other intriguing aspects of twinning. Until recently, mothers were all expected to gain under forty pounds during pregnancy, no matter if she was carrying a single or multiple pregnancy. Now, doctors and medical experts have come up with what they consider an "ideal twin outcome." Twins are supposed to have a slightly shorter gestation period lasting anywhere from thirty five to thirty eight weeks. Triplets are even less showing an average for ideal outcome at thirty-four to thirty-five weeks. Also, the mean birth weights attributed as ideal are 2500-2900 grams per infant. Mothers with ideal twin outcomes gained forty-five pounds on average and mothers of triplets with ideal outcomes gained fifty to fifty-five pounds. Mothers with non-ideal outcomes gained less than thirty-five pounds (8).

Furthermore, research shows that in industrialized countries, the rate of mothers having multiple births is rising. This is most likely due to the fact that mothers are waiting longer for pregnancy and widespread use of technology to help reproduction. There are today one hundred and twenty five million living multiples in the world. In the industrialized US, there were 106,689 multiples born in 1996, 100,750 of them being twins, 5,298 triplets, 560 quadruplets, and 81 higher level pregnancies (8). The fact that mothers wait until a later age before having children makes the incidence of multiple birth even more high risk. All multiple pregnancies are high risk compared to singleton pregnancies and risks to the fetuses and the mother increase with the number of infants the mother is carrying. The risks include an increased occurrence of pre-term labor and low birth weights (which both are already less than singleton births). Also there is an increased risks of miscarriage. In fact, some researchers believe that a good number more of human pregnancies may begin as twins but the higher loss rate reduces the multiple pregnancy to a singleton pregnancy or in some cases a complete miscarriage. Professor C. Bocklage of East Carolina University School of Medicine believes that one in eight conceptions begin as twins and that for every twin pair born alive, ten to twelve originally multiple pregnancies result in a singleton birth (10).

Mothers have a one in 150 chance of having fraternal twins and if they have already had fraternal twins, that chance increases to one in forty. For a woman whose mother has had fraternal twins, her chance of having fraternal twins is one in ninety. This is because the rate fraternal twins is affected by genetic factors. The rate of identical twins, on the other hand, do not appear to be influenced at all by hereditary factors (1). The incidence of fraternal twins, since it is affected by genetics, seems to vary by race. People of African decent have the highest rate of fraternal twinning showing one out of seventy births as twins. Caucasians are next with one out of eighty-eight births, then Japanese with one out of 150, and finally Chinese with one out of 300. One particular tribe in Africa has a particularly high rate of twinning. The Yoruba tribe of Nigeria exhibits one out of forty five births as fraternal twins. Also, in some places, like Japan for example, there seems to be a seasonal influence on the rate of fraternal twinning. This variation differs from country to country and in some places, like the Netherlands, the seasonal variations of twin births are parallel to those of singleton births showing that in these cases, the difference is probably due to cultural and not biological factors (8).

The study of twins, or gemellology, is an important aspect of biology. By examining twins, researchers gain knowledge of genetic and environmental factors that effect the growth and development of all people. Also, the statistical information taken on twins provides information on the influence of new technology and cultural differences (such as the average age of pregnant women) on pregnancies. Most importantly, the studies conducted on twins are intriguing in themselves providing insight into a world of twins that has interested humans for ages.

Bibliography and WWW Sources

1) Abbe, Kathryn McLaughlin, and Frances McLaughlin Gill. , Twins on Twins. New York: Crown Publishers, 1980.

2) Bank, Stephen P. and Kahn, Micheal D., The Sibling Bond. New York: Basic Books Publishers, 1941.

3) Kitcher, Philip , The Lives to Come. New York: Simon and Shuster,1996.

4)Koch, Helen L., Twins and Twin Relations. Chicago: The University of Chicago Press,1966.

5) Newman, Horatio, Frank N. Freeman, and Karl J. Holzinger., Twins: A Study of Heredity and Environment. Chicago: The University of Chicago Press, 1965.

6) Strickberger, Monroe W., Genetics. New York: The Macmillian Company, 1968.

7) Twin Biology, Twins Reared Apart, Conjoined Twins.Cited December 12, 2000

8) Center for Study of Multiple BirthCited December 12

9) The Twin to Twin Transfusion Syndrome Foundation , Cited December 15, 2000

10) Occurance and Twinfacts Cited December 12, 2000



Continuing conversation
(to contribute your own observations/thoughts, post a comment below)

11/07/2005, from a Reader on the Web

I have had fraternal twins that shared the same placenta in my genetics classroom and not only were they not impaired as in this article but were very vigorous, healthy and every way. They were top students and both made it in the two different military academies. As sophomores in high school their uncanny truly identical features made it difficult for all but their mother and father.The only feature that was different was that one was a little more competitive. But it was very hard to see. I believe that sharing identical food, hormones etc. in the womb imprinted them for many years. Greg

Additional comments made prior to 2007
I am the mother of monochorionic female twins born in 1987. I was told that since they shared the chorion that they were "identical" and accepted that as truth. As infants, they were impossible to identify and I kept a hospital ID bracelet on their opposite wrists for several months. Over the years I have come to question that diagnosis for the following reasons: One twin has 3 lower incisor teeth while the other has the normal number of 4. They each had 3 wisdom teeth but in different positions. One twin is slightly taller than her sister but their frame structure is identical (and quite the same as mine). Their shoe sizes are different also. Their facial features while very similar are still different enough to make identification fairly easy since puberty. Moles and birthmarks are altogether different. One twin is right handed and the other is left. Socially they are close but not as close as I would have expected. It's only recently that another mother of twins told me of a 3rd type of twinning. I have often wondered about these things and finally took the time to research it. Thanks for giving me the opportunity to share ... Dianne, 24 November 2006


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