Two babies born to a mother at one birth. There are two types of twins, monozygotic and dizygotic. Members of a twin pair are called co-twins.
Controversy surrounding the definition of a twin arose with the advent of reproductive technologies enabling the simultaneous fertilization of eggs, with separate implantation. The unique “twinlike” relationships that would result between parents and cloned children (who would be genetically identical to their parents) also challenge current conceptions of twinship. Monozygotic twins are clones (genetically identical individuals derived from a single fertilized egg), but parents and cloned children would not be twins for several reasons, such as their differing prenatal and postnatal environments. See alsoReproductive technology.
Monozygotic twins
The division of a single fertilized egg (or zygote) between 1 and 14 days postconception results in monozygotic twins. They share virtually all their genes and, with very rare exception due to unusual embryological events, are of the same sex. A common assumption is that because monozygotic co-twins have a shared heredity, their behavioral or physical differences are fully explained by environmental factors. However, monozygotic twins are never exactly alike in any measured trait, and may even differ for genetic reasons.
Sometimes chromosomes fail to separate after fertilization, causing some cells to contain the normal chromosome number (46) and others to contain an abnormal number. This process, mosaicism, results in monozygotic co-twins who differ in chromosomal constitution. There are several other intriguing variations of monozygotic twinning. Splitting of the zygote after day 7 or 8 may lead to mirror-image reversal in certain traits, such as handedness or direction of hair whorl. The timing of zygotic division has also been associated with placentation. Monozygotic twins resulting from earlier zygotic division have separate placentae and fetal membranes (chorion and amnion), while monozygotic twins resulting from later zygotic division share some or all of these structures. Should the zygote divide after 14 days, the twins may fail to separate completely. This process, known as conjoined twinning, occurs in approximately 1 monozygotic twin birth out of 200. The many varieties of conjoined twins differ as to the nature and extent of their shared anatomy. Approximately 70% of such twins are female. There do not appear to be any predisposing factors to conjoined twinning. See alsoMosaicism.
Dizygotic twins result when two different eggs undergo fertilization by two different spermatozoa, not necessarily at the same time. Dizygotic twins share, on average, 50% of their genes, by descent, so that the genetic relationship between dizygotic co-twins is exactly the same as that of ordinary brothers or sisters. Dizygotic twins may be of the same or opposite sex, outcomes that occur with approximately equal frequency.
There are some unusual variations of dizygotic twinning. There is the possibility of polar body twinning, whereby divisions of the ovum prior to fertilization by separate spermatozoa could result in twins whose genetic relatedness falls between that of monozygotic and dizygotic twins. Blood chimerism, another variation, refers to the presence of more than one distinct red blood cell population, derived from two zygotes, and has been explained by connections between two placentae. In humans, chimerism can occur in dizygotic twins. Superfecundation is the conception of dizygotic twins following separate fertilizations, usually within several days, in which case each co-twin could have a different father. Superfetation, which refers to multiple conceptions occurring several weeks or even one month apart, may be evidenced by delivery of full-term infants separated by weeks or months and by the birth or abortion of twin infants displaying differential developmental status. See alsoOogenesis.
Epidemiology
According to conventional twinning rates, monozygotic twins represent approximately one-third of twins born in Caucasian populations and occur at a rate of 3–4 per 1000 births. The biological events responsible for monozygotic twinning are not well understood. It is generally agreed that monozygotic twinning occurs randomly and not as a genetically transmitted tendency. Some recent evidence from Sweden suggests an increased tendency for mothers who are monozygotic twins to bear same-sex twins themselves; further work will be needed to resolve this question.
Dizygotic twinning represents approximately two-thirds of twins born in Caucasian populations. The dizygotic twinning rate is lowest among Asian populations (2 per 1000 births), intermediate among Caucasian populations (8 per 1000 births), and highest among African populations (50 per 1000 births in parts of Nigeria). The natural twinning rate increases with maternal age, up to between 35 and 39 years, and then declines. Dizygotic twinning has also been linked to increased parity, or the number of children to which a woman has previously given birth. Mothers of dizygotic twins are also significantly taller and heavier, on average, than mothers of monozygotic twins and singletons. Dizygotic twinning appears to be genetically influenced, although the pattern of transmission within families is unknown.
Twinning rates have risen dramatically since about 1980 mainly due to advances in fertility treatments (for example, in vitro fertilization and ovulation induction), but also due to delays in the child-bearing years. The increase has mainly involved dizygotic twinning in which multiple ovulation and maternal age matter.
Twins are of two types: monovular (identical), from the union of one sperm and one ovum, and binovular (non-identical) resulting from the fertilization of two separate ova. The cell produced by fertilization is called a zygote (from the Greek for ‘yoked’), so they are also known as monozygotic and dizygotic. Dizygotic twins are physically and genetically as dissimilar as any siblings. Monozygotic twins, having resulted from the cleavage of a single ‘conceptus’ — the splitting and separation of an early embryo — are therefore, with rare exceptions, genetically identical.
The incidence of multiple pregnancies varies in different racial groups. To quote ‘Hellin's law’ (1895): ‘twins occur in 1/89 births, triplets 1/ (89) 2, quadruplets 1/(89) 3 and so on’. The formula is roughly correct, although twins occur in Caucasians 1/80 to 1/90, in Asiatics 1/150 or less, and black Africans 1/50 with the highest incidence of twinning amongst the Yoruba people of Nigeria for whom 1 in 25 births are of twins. It is the rate of non-identical (dizygotic) twinning that varies around the world: identical (monozygotic) twins occur at a similar rate of 1 in 300 births in all populations. These statistics are based on clinical findings in viable pregnancies. However the initial ‘hidden’ twinning rate is probably higher: with increasing use of ultrasound in early pregnancy it is found that before 12 weeks one of the twins may die and be absorbed leaving an apparent singleton. In Australia the rate of twinning has increased approximately 25% over the past 20 years, partly due to a significant increase in the percentage of births to women aged 35 and over, and partly to the treatment of infertility by ovulation stimulation or assisted conception by gamete intra-fallopian transfer (GIFT) or in-vitro fertilization (IVF).
Twin pregnancy is more prone to complication than single pregnancies and possible hazards of premature birth and poor growth in the womb necessitate increased antenatal surveillance. If twins are identical and they share a single placenta, one baby can steal blood from the other, causing a condition known as ‘Twin- twin transfusion syndrome’.
Multiple pregnancies carry a greater risk of losing a baby before, during, or after birth than singleton pregnancies: multiple pregnancies overall account for more than 10% of all perinatal deaths; the greater the number, the greater the risk. Cerebral palsy in survivors is six times more common in twins than singletons.
The birth of twins has been a source of fascination in many cultures throughout history and the twin image has been incorporated in myths, folklore, and religions. The Old Testament of the Bible tells of Isaac's wife, Rebekah, who eventually conceived after nineteen years of marriage. Twin boys were born. The first was red and hairy, and he was named Esau, meaning ‘red’. His brother was born holding Esau's heel and so he was called Jacob, meaning ‘he who grasps the heel’. Ancient Rome was founded, according to legend by Romulus and Remus, the twin progeny of Mars, god of war, and a mortal princess. In some African communities, twins were regarded with great favour; in others, with great suspicion. The Yoruba in Nigeria were well aware of the high mortality associated with twinning in the past, and they made small wooden sculptures, ‘ibeji’ that had spiritual significance if one of twins died.
Two siblings produced in the same pregnancy and developed from one egg (identical, monozygotic) or from two eggs fertilized at the same time (fraternal, dizygotic).
Twins occur when two babies are born at the same birth.
Description
Identical, or monozygotic, twins are of the same sex and are genetically identical and physically similar, because they both come from one ovum (egg), which, after fertilization, divides in two and develops into two separate fetuses. Fraternal, or dizygotic, twins occur when the mother produces two eggs in one monthly cycle and both eggs are fertilized. The conceptions may take place on two separate occasions and could involve different fathers.
Fertilized egg division which produces twins can either happen early or late in development. In the case of early separation, the two fetuses either share an amniotic sac or each has a separate amniotic sac. If the fetuses share an amniotic sac, they also share a placenta. If the two fetuses have separate amniotic sacs, they can either share a placenta or have two separate placentas. Twins can also result from a fertilized egg that divides slightly later in development. In this case, the twins share an amniotic sac and a placenta. It is from these cases of late separation that conjoined (Siamese) twins sometimes develop.
Fraternal twins, who are no more genetically alike than ordinary siblings, may be of the same or different sex and may bear some similarity of appearance. Fraternal twinning appears to be passed on by the female members of a family. If the mother is a fraternal twin herself, has fraternal twin siblings, or fraternal twin relatives on her side of the family, she is more likely to give birth to fraternal twins. If she has already given birth to fraternal twins, her chances of giving birth to fraternal twins again are four times greater than those of a woman who has not had fraternal twins. In vitro fertilization increases a woman's chances for having multiple birth.
The number of twins born in the United States rose between the early 1980s and the early 2000s. In 1980, there were 69,339 sets of twins born, and in 2002 there were 125,134 sets of twins born in the United States. According to data gathered by the Centers for Disease Control (CDC), there is considerable variation among the states in number and rate of twin births. In 1994, for example, the twin birth rate ranged from 19.8 per 1,000 live births in Idaho and New Mexico to 27.7 per thousand in Connecticut and Massachusetts. One factor that may influence the distribution of multiple births is whether the state provides insurance coverage for procedures such as in vitro fertilization (IVF) and other treatments to improve fertility. These procedures increase the chance of multiple births.
Ethnicity is another factor that may correlate to the twin birth rate. For 1994, the twin birth rate among non-Hispanic white mothers was 24.3 per 1000 live births; among non-Hispanic black mothers, 28.3 per 1000; and among Hispanic mothers, 18.6 per 1000. There are also significant differences internationally in the number of twins born with the rate in Belgium almost six times the rate in China.
The CDC also studies whether maternal age has any correlation with the rate of twin births. The data seem to suggest that mothers in states with rates of twin births higher than the overall rate for the United States are older on average, and mothers in states with rates of twin births lower than the overall rate for the United States are younger. Again, as in vitro fertilization is more widely done, the incidence of multiple births will increase.
Infancy
Parents should avoid giving twins very similar names. Twins should be treated as two individuals and not as a package. They may need to be fed at different times and may develop skills at different rates. It is important to spend time with each twin separately so that they become used to being separated from each other for short times and know that they are each valued as individuals.
Toddlerhood
To help twins understand who they are as individuals, parents should avoid dressing both twins the same. It is preferable that each child receive toys that are geared towards their individual interests rather than each receiving the same toy.
School Age
Sibling rivalry can be more intense in twins than in siblings of different ages. This is not unusual, because teachers, coaches, and even parents tend to compare twins. All children compare themselves to their siblings, and having others do this regularly can add to the pressure and stress of being a twin. Parents should consider arranging to have the twins put in different classes in school to help foster individuality. Each twin will probably have different skills, interests, and friends, and they should be encouraged to peruse activities separately if their interests diverge. Helping teachers, coaches, babysitters, and friends understand that it is important to treat the twins as two separate people can be very important. Friends should be encouraged to give separate gifts for birthdays and holidays, taking each child's special interests and talents into account.
Common Problems
Twins often have a harder time developing their own independent identities than other children. Twins are more likely to have low birth weights or be delivered prematurely than single babies.
Parental Concerns
Raising twins can be more challenging than raising two single children. The children may need to eat, sleep, and be changed at different times when they are infants. It can also be more expensive, because things like car seats and cribs must be purchased at the same time instead of reused for the second child. Some stores have special discounts for parents of twins.
When to Call the Doctor
Parents should call the doctor if one or both of their children seems ill, just as they would for any other child or children.
Resources
Books
Noble, Elizabeth, with Leo Sorger. Having Twins and More: A Parent's Guide to Multiple Pregnancy, Birth, and Early Childhood, 3 ed. Boston, MA: Houghton Mifflin, 2003.
Pearlman, Eileen M., and Jill A. Ganon. Raising Twins: What Parents Want to Know, and What Twins Want to Tell Them. New York: Harper Resource, 2000.
Periodicals
Brown, Judith E., and Marcia Carlson. "Nutrition and Multifetal Pregnancy." Journal of the American Dietetic Association 100 (March 2000): 343.
Organizations
National Organization of Mothers of Twins Clubs. PO Box 438, Thompsons Station, TN 37179–0438. Web site: www.nomotc.org.
To estimate the relative contributions of genes and environment to the cause of disease by comparing MZ to DZ concordance;
To investigate environmental determinants of etiology in discordant twin pair studies;
To investigate environmental influences on disease course in concordant twin pair studies;
To characterize "presymptomatic" or "at risk" states by studying the unaffected twins in discordant pairs.
Twins are siblings carried together in the womb and born at the same time. Similarities and differences between twins can be used to answer questions about the role genes and the environment play in the development of traits such as personality, intelligence, and susceptibility to disease. While results from any single pair of twins cannot provide conclusive answers to such questions, the study of large numbers of twin pairs allows researchers to draw conclusions about inheritance with a significant degree of confidence.
Developmental Mechanisms
Twins are classified as either dizygotic or monozygotic. Dizygotic twins (also called fraternal twins) arise from two separately fertilized eggs, or zygotes. In humans, usually only one egg is released at a time from a woman's ovaries. When two are released, both may become fertilized by separate sperm and implant in the uterus. Dizygotic twins develop separate placentas and amniotic sacs. They may be of the same or different sexes. In the absence of reproductive technology interventions, dizygotic twinning occurs in approximately three of every thousand human births, a rate that increases with maternal age, varies with ethnic group, and is probably influenced by genes that control pituitary function. Various types of assisted reproductive technologies routinely create dizygotic twins, triplets, and higher numbers of offspring.
Monozygotic twins (also called identical twins) arise from a single fertilized egg. At some point after the zygote begins to divide, the cell mass splits into two, creating two embryos from one. Monozygotic twinning occurs in approximately 0.25 percent of human births. Monozygotic twins are always of the same sex. If the cell mass splits before about day five after fertilization, the two embryos will develop with separate placentas and separate amniotic sacs. This occurs in about two-thirds of human monozygotic twins. Between day five and about day nine, splitting leads to two amniotic sacs but one placenta. This occurs in about one-third of Monozygotic twins. Twins that split after day nine will share the amniotic sac. Splitting that late also increases the likelihood that the twins will not separate completely and will develop into conjoined (Siamese) twins.
Monozygotic Versus Dizygotic Twins
Because monozygotic (MZ) twins develop from a single fertilized egg, they begin life with exactly the same set of genes. In this respect, they are clones—organisms whose genes are identical. As discussed below, however, they may accumulate genetic and other differences during development.
In contrast, dizygotic (DZ) twins are no more genetically close than any pair of siblings. While it is commonly said that siblings share half their genes, this is incorrect for two reasons. First, the random nature of meiosis and fertilization means that two siblings could end up with many, or few, genes from a particular parent in common. Second, there are many human genes for which there is only one common form, or allele. Therefore, any two people will share many alleles, regardless of their relationship. Only those genes with more than one allele form the basis of human genetic variation. These are the real focus of the question about gene-sharing in siblings. Of these variable genes, siblings (including dizygotic twins) on average share half.
Because dizygotic twins are the same age, they may share more of their environment than would two siblings of different ages. For instance, because they are likely to be engaged in similar activities, dizygotic twins are more likely to have similar environmental exposures (including behaviors, diet, hobbies, exposure to infectious agents, and exposure to chemicals)—whether at home, at school, or in the community—than two siblings of different ages and different activity patterns. It is this similarity of environment but difference of genes that makes them a useful contrast to monozygotic twins, whose environments and genes are largely identical.
Similarities and Differences Between Monozygotic Twins
The fertilized egg cell that gives rise to MZ twins begins life with a single set of genes, and so we might predict that every cell that arises from it would be exactly identical. However, small differences between daughter cells may accumulate throughout embryonic development and later in life. The earliest difference may be in the mitochondria each inherits. Mitochondria are the cell's power plants and contain a small amount of DNA. Some of the hundreds of mitochondria in a cell may contain mutations. If the cells that create the two twins carry different mitochondrial genes, even identical twins will be genetically different. Mutations can also accumulate during embryonic development, or after birth, either in the mitochondrial genes or the genes in the nucleus. Such mutations may have a significant effect: Some types of cancer are due to mutations accumulated during one's lifetime, often through exposure to environmental chemicals or radiation.
For the vast majority of genes, though, MZ twins are exactly identical. Nonetheless, twins do experience slightly different environments, even when reared together, and any early differences between them may be accentuated by families members, or by one another, leading to development of very different personalities.
Amazing Twin Similarities
Some of the most tantalizing clues to the genetic basis of human personality and behavior come from studies of MZ twins reared apart since birth. Such twins have the same genes but, presumably, different environments. A major study of more than 100 such twin pairs showed some remarkable coincidences. A pair of twins meeting for the first time at age thirty-nine each arrived wearing seven rings, two bracelets on one wrist, and a watch and one bracelet on the other wrist. Another twin pair discovered they each had dogs named Toy, had married and divorced women named Linda, remarried women named Betty, and named their sons James Allan and James Alan.
Table 1
CONCORDANCE IN TWIN STUDIES
Pairwise concordance
Number of twin pairs in which both are affected Total number of twin pairs
Proband-wise Concordance
[2c2+ c1] [2c2+ c1+ d]
A proband is an independently ascertained twin with the disease; independently ascertained means the twin was NOT identified through the co-twin.
c2 = the number of concordant pairs in which both twins are probands
c1 = the number of concordant pairs in which only one twin is a proband
d = the number of discordant pairs
Using concordance patterns to estimate the relative contributions of genetic and environmental determinants to a condition or disorder:
If MZ concordance = 100%
Only genetic determinants likely
If MZ > DZ concordance
Genetic determinants important
Environmental modifiers likely
If MZ concordance = DZ concordance
Shared environmental determinants likely
While these coincidences are amazing, it is important to remember that many more twin pairs in this study did not have such parallel lives or habits. Such stories are curious and provocative but cannot by themselves tell us about the relative contributions of genetics and the environment in shaping personality, behavior, health, or other aspects of the self.
Twin Studies and Concordance
Insight into such questions can be gleaned by several types of studies that compare twins. Comparison of MZ twins reared apart is one type of study but is hampered by the extreme rarity of such twin pairs. Another type of study, comparing MZ twins to DZ twins, is more commonly done, because there are many hundreds of thousands of such twin pairs worldwide. Data on twins have been collected by numerous research groups who have created large and growing databases (registries) that can be mined for information.
Determining a characteristic called concordance plays a crucial role in most such studies. A twin pair is said to be concordant for a trait if both members show it. If neither twin shows the trait, the pair is also concordant, but for the absence of the trait. For instance, twins are concordant for Alzheimer's disease if both develop it. They are discordant if one does have the disease but the other does not.
Table 2
PAIRWISE CONCORDANCE FOR PARKINSON'S DISEASE
Concordant Pairs
Discordant Pairs
Pairwise Concordance
Risk of Concordance if MZ
MZ
DZ
MZ
DZ
MZ
DZ
RR (95% CI)
Overall/span>
11
10
60
80
15.5%
11.1%
1.39(0.63-3.10)
First twindiagnosed <50
4
2
0
10
100.0%
16.7%
6.00 (1.69-21.3)
First twin diagnosed >50
7
8
58
68
10.8%
10.5%
1.02 (0.39-2.67)
If a trait is strongly influenced by genes, more MZ twin pairs should be concordant than DZ twin pairs, because MZ twins share more genes. Comparing concordance rates between the two groups, and applying some mathematical analysis, allows researchers to estimate the genetic contribution to a trait, as shown in Table 1.
Twin Registries
Twin studies can have several starting points. Some investigators begin simply by trying to identify twins who will volunteer to be part of a particular research study. Often twins are sought by advertising for twins with the particular disease of interest. This approach has the advantage of simplicity, as twins identify themselves to the research team.
However, twins who volunteer may differ in some important way from those who do not volunteer, and this could affect the conclusions drawn from the study. For example, MZ twins are more likely to volunteer, in general, than DZ twins are. This tendency to volunteer for twin studies among MZ twins is probably because being a twin is a more central part of the identity of MZ pairs than DZ pairs. Also, twins concordant for a particular disease are more likely to volunteer than those without the disease are. If both influences are at work in the same study, more concordant MZ twins than DZ twins may be identified, not because there is an actual difference in concordance between MZ and DZ twins (and thus a genetic effect at work), but because more concordant MZ twins volunteered for the study. If this pattern of volunteerism is mistaken to represent the true pattern of the disease in all twins, an inappropriate conclusion that the disease has genetic causes could result.
Other twin registries attempt to identify all twins within a particular population. One approach is the statewide or national twin registry. All twin births in the region are reported to a central registrar. This results in a more complete picture of all twin pairs in these populations. Examples include the statewide Virginia and Minnesota twin registries in the United States and many national twin registries, including those in the United Kingdom, Australia, the Scandinavian countries, Germany, Belgium, the Netherlands, Italy, and Sri Lanka.
Twin registries have also been assembled from among special populations. Examples in the United States are registries assembled from military records (the World War II Veteran Twins Registry and the Vietnam Era Twin Registry) and from Medicare files (the U.S. Registry of Elderly African-American Twins). In these registries, likely adult twins were identified by searching records to identify individuals with identical dates of birth, birthplaces, and surnames. These individuals were then contacted to verify whether they actually constituted a twin pair. Registries may also be established by identifying twin births within a health maintenance organization (such as the Kaiser Permanente Twin Cohort, in California).
Each registry varies in the amount of contact with registrants. In all, individual contact is strictly monitored to preserve the privacy of each twin. Every research proposal must be approved by a panel to assure the scientific value of the project, the justification for doing the study in twins, and to ensure that the privacy and safety of individual twins will be protected.
Twin registries can be useful starting points for investigating many questions about the genetic and environmental determinants of a trait. Records linkage studies involve no personal contact with the twins. Instead, information in the twin registry is "linked" electronically to information in another database, such as a national health insurance database or a cancer registry. In this way, twins with a particular health problem can be identified, and concordance estimates can be calculated. Similarly, information collected for each twin at registration can later be used to investigate certain kinds of questions without ever contacting the individual twins. On the other end of the spectrum, twins may be asked to volunteer for physical examinations, blood tests, radiological studies, or interviews. Depending on the questions asked, such studies may be useful for comparing concordance, or for identifying risk factors or modifying factors for a trait.
Twin Studies to Investigate the Cause of Parkinson's Disease
An example of the use of investigations in twins to understand more about a disease is provided by recent work in Parkinson's disease. Parkinson's disease (PD) is a progressive neurodegenerative disease causing slowness, tremor, and problems with walking and balance. PD is rare before age fifty but becomes more common thereafter, with increasing age. The cause of PD has long been debated. Both genetic and environmental causes have been suggested, but neither has been definitively shown. Researchers turned to studies in twins to determine the relative contribution of genes and environment to the disease.
The first studies identified twin pairs by recruiting through physicians and PD patient organizations. Studies in the United States, the United Kingdom, and Germany identified 103 pairs, of which only thirteen were concordant for PD. In Finland, forty-two twins with PD were identified by records linkage, but among these was only one concordant pair—a DZ pair. No study had convincingly demonstrated greater monozygotic than dizygotic concordance for the disease, and in all studies the preponderance of twin pairs were discordant for disease. These findings supported an environmental cause of PD. Nonetheless, the advent of molecular genetics prompted great interest in investigations of genetic causes of disease and prompted the resurgence of the hypothesis that all PD had a genetic cause. To address this, a study in a large, unselected cohort—the National Academy of Sciences/National Resource Council (NAS/NRC) World War II Veteran Twins Registry—was undertaken.
In the mid-1950s, the Medical Follow-up Agency of the Institute of Medicine of the NAS/NRC established a registry of approximately 32,000 Caucasian male twins, all of whom were born between 1917 and 1927 and were veterans of the U.S. Armed Services. In all, 161 twin pairs were identified, twenty-one of which were concordant for PD, as shown in Table 2. In those few pairs with early-onset PD, concordance was greater in MZ pairs. In those with more typical PD, beginning after age fifty, there was no difference in MZ and DZ concordance.
These findings suggest a strong genetic determinant for early-onset disease but predominantly environmental causes in more typical late-onset disease. One caveat is the narrow age range of the twins, who were sixty-seven to seventy-seven years old when studied. Since PD is a late-life disorder, PD in some twins may have been missed with an examination at only one time point. To overcome this, a second evaluation is in progress.
Risk-Factor Investigations in Twins
Studies of twin pairs discordant for disease can be useful for identifying risk factors for disease. Since both genetic and environmental factors are extensively shared by twins, particularly by MZ twins, case-control studies can be particularly powerful. In such a study, each twin is interviewed with regard to specific environmental factors—such as occupation, lifestyle factors, illnesses or injuries, and diet—prior to the onset of the disease in the affected twin. The presence of these factors in the twin with the disease is compared to the twin without disease. An association of an environmental factor with the disease suggests this factor may be causally related. Factors more common among the unaffected twins suggest that the factor may protect against the development of the disease.
Environmental influences on PD have been investigated by studying discordant twin pairs. PD has repeatedly been found to be more common in people who do not smoke cigarettes. Some have proposed that some people are genetically predisposed to both Parkinson's disease and smoking, while others suggest cigarette smoking somehow prevents the degeneration that leads to PD. In two studies of discordant twin pairs, cigarette smoking was more common in the twin without Parkinson's disease, especially in the MZ pairs. Because monozygotic twins are genetically identical, this pattern tips the scales in favor of a direct biological action of cigarette smoke.
As medicine focuses more on early intervention or prevention, it becomes important to identify those persons at risk for a particular condition. This can be a problem if there is no diagnostic test. In discordant twin pairs, the unaffected twin is more likely to be "at risk" for a particular condition, whether due to shared genes or environment, than would be true for two nontwins. Therefore, studying the unaffected "at risk" twin may help to clarify what features are useful for predicting those who later will develop a particular disease. For example, in the PD twin study, the unaffected twins are being studied prospectively with brain imaging tests that may show early evidence of injury to the brain area damaged in PD. If abnormalities on this test are found to precede the development of PD, this could provide a useful method of early detection. When treatments to slow or stop the onset of PD are available, individuals with imaging abnormalities may receive intervention before symptoms develop.
Results from Twin Studies of Other Disorders and Conditions
The twin study method has been used to try to determine the extent of genetic or environmental influence on a wide variety of traits and conditions. Among these are sense of humor, which appears to be largely environmentally determined, as MZ and DZ pairs have similar concordance. Examples of other diseases in which MZ concordance exceeds DZ concordance, suggesting a significant genetic component, include addictive behaviors such as cigarette smoking and alcohol drinking, mental illnesses such as schizophrenia, as well as stroke and certain types of high blood pressure. Twin studies of many other disorders are ongoing.
Conclusion
Twin studies provide a unique approach to investigating the determinants of a disease or condition. A single twin study cannot absolutely determine the importance of genetic or environmental factors. However, the twin study method, in combination with other approaches, can be a powerful tool for unraveling the causes of disease.
Bibliography
Bouchard, T. J., et al. "The Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart." Science 250 (1990): 223-228.
Segal, Nancy L. Entwined Lives: Twins and What They Tell Us about Human Behavior. New York: Plume, 2000.
Wright, Lawrence. Twins: And What They Tell Us about Who We Are. New York: John Wiley & Sons, 1997.
It has long been believed that there is a special relationship between identical twins, a belief that has become the subject of contemporary research from a variety of approaches. Research has suggested that there are startling correspondences between twins' temperaments, personalities, lifestyles, and even sensitivity to names.
In 1979, the University of Minnesota began a study of identical twins in which twins separated for years were investigated and subjected to medical and psychological tests. The results of nine identical twin studies, involving over 15,000 questions, demonstrated affinities between the subjects.
For example, unknown to each other, Jim Spring and Jim Lewis were raised in different Ohio towns. Both married and divorced women named Linda and chose women named Betty as second wives. Each of the two Jims named his son James Allan and had a favorite dog named Toy. Both twins had remarkable similarities in medical profiles, including identical blood pressures and sleep and heartbeat patterns. Both also suddenly put on 10 pounds at the same time in their lives. At the age of 18, both Jim twins suffered similar syndromes of intermittent migraine headaches. Their drinking and smoking habits were also identical, and both chewed their fingernails.
Another pair of identical twins, Jack and Oscar, were raised apart with completely different backgrounds. Jack was brought up as an American Jew by his father after his parents separated; the mother took Oscar back to Germany (where she had been born) where he was raised as a Catholic, later joining the Nazi Youth party. In adult life, Jack ran a store in San Diego, while Oscar became a factory supervisor in Germany. But both men wore wire-rimmed eyeglasses and mustaches and two-pocket shirts with epaulets. Both were absentminded and had other matching idiosyncracies, such as storing rubber bands on their wrists.
Bridget and Dorothy were identical British twins who were raised apart after being separated soon after birth, yet when they met in 1941, each wore two bracelets on one wrist, and a watch and bracelet on the other. Each sister also wore seven rings. Each twin had married and had a family of a boy and a girl. The sons had been christened Richard Andrew and Andrew Richard, while the daughters were Karen Louise and Catherine Louise.
Many such identical twins share IQ and psychological profiles, as well as EEG tracings. It is not yet clear whether the coincidences derive from some kind of psychic bonding or simply indicate some manifestation of inheritance. It should be noted that astrologers have investigated twins, with ambiguous results to date, with the idea of verifying and informing astrology.
Sources:
Watson, Peter. Twins: An Investigation Into the Strange Coincidences in the Lives of Separated Twins. London: Hutchinson,1981.
The general term for more than one offspring from the same pregnancy is multiples, for example triplets refers to cases
of three offspring from the same pregnancy. A fetus alone in the womb is called a
singleton.
Human twins are two individuals who have shared the uterus during a single pregnancy and are
usually, but not necessarily, born in close succession. Due to the limited size of the
mother's womb, multiple pregnancies are much less likely to carry to full term than singleton births, with twin pregnancies
lasting only 37 weeks on average, 3 weeks less than full term.[citation needed] Since premature births can have
health consequences for the babies, twin births are often handled with special precautions.
There are estimated to be approximately 125 million human twins and triplets in the world (roughly 1.9% of the
world population), and just 10 million identical twins (roughly 0.2% of the world
population and 8% of all twins).[citation needed]
Twins can either be monozygotic or dizygotic.
Types of twins
There are five variations of twinning that commonly occur. The three most common variations are all fraternal: (1) male-female
twins are the most common result, at about 40% of all twins born; (2) female fraternal twins (sometimes called sororal twins);
(3) male fraternal twins. The last two are identical: (4) female identical twins and (5) (least common) male identical twins.
Male singletons are slightly, about 5%, more common than female singletons. However, males are also more susceptible than females
to death in utero, and since the death rate in utero is higher for twins, it leads to female twins being more common than male
twins.
Another variety of twins, "polar body twins," (one egg fertilized by two different sperm) is a phenomenon that was
hypothesized to occur and may recently have been proven to exist. Polar body twinning would result in "half-identical"
twins.[1]
Eight month old fraternal twin girls napping.
Fraternal twins
Fraternal twins (commonly known as "non-identical twins") usually occur when two fertilized eggs are implanted in the uterine wall at the same time. The two eggs form two zygotes, and
these twins are therefore also known as dizygotic as well as "biovular" twins. When two eggs are independently fertilized
by two different sperm cells, fraternal twins result.
Dizygotic twins, like any other siblings, have an extremely small chance of having the exact same chromosome profile.
Like any other siblings, fraternal twins may look very similar, particularly given that they are the same age. However, fraternal
twins may also look very different from each other. They may be different sexes or the same sex. The same holds true for brothers
and sisters from the same parents, meaning that fraternal twins are simply brothers and/or sisters who happen to have the same
age.
Studies show that there is a genetic basis for fraternal twinning. However, it is only the female partner that has any
influence on the chances of having fraternal twins as the male cannot make her release more than one ovum. Fraternal twinning
ranges from 6 per thousand births in Japan (similar to the rate of identical twins) to 14 and more per thousand in some
African states.[citation needed]
Fraternals are also more common for older mothers, with twinning rates doubling in mothers over the age of 35.[citation needed] With the advent of technologies and
techniques to assist women in getting pregnant, the rate of fraternals has increased markedly. For example, in New York City's
Upper East Side there were 3,707 twin births in 1995; there were 4,153 in 2003; and there were 4,655 in 2004. Triplet births have
also risen, from 60 in 1995 to 299 in 2004.
Identical twins
Identical twins.
Identical twins occur when a single egg is fertilized to form one zygote (monozygotic) which then divides into two separate embryos.
Although their traits and physical appearances are not exactly the same due to environmental conditions both in and outside the
womb, they do have identical DNA. This is not considered to be a hereditary trait, but rather an anomaly that occurs in birthing
at a rate of about 3 in every 1000 deliveries worldwide,[2]
regardless of ethnic background. The two embryos develop into fetuses sharing the same womb. When
one egg is fertilized by one sperm cell, and then divides and separates, two identical cells will result. If the zygote splits
very early (in the first 2 days after fertilization) they may develop separate placentas (chorion) and separate sacs (amnion).
These are called dichorionic, diamniotic (or "di/di") twins, which occurs 20-30% of the time. Most of the time in identical twins
the zygote will split after 2 days, resulting in a shared placenta, but two separate sacs. These are called monochorionic,
diamniotic ("mono/di") twins.
In about 1% of identical twins the splitting occurs late enough to result in both a shared placenta and a shared sac called;
monochorionic, monoamniotic ("mono/mono") twins. Finally, the zygote may split extremely late, resulting in conjoined twins. Mortality is highest for conjoined twins due to the many complications resulting from
shared organs. Mono/mono twins have an overall in-utero mortality of about 60%, principally due to cord entanglement prior to 32
weeks gestation. Many times, monoamniotic twins are delivered at 32 weeks electively for the safety of the babies. In higher
order multiples, there can sometimes be a combination of fraternal/identical twins.This is a set of Quads with a set of mono/mono twins and a set of di/di twins.
Mono/di twins have about a 25% mortality due to twin-twin transfusion. Di/di twins have the lowest mortality risk at about 9%,
although that is still significantly higher than that of singletons.[3]
Monozygotic twins are genetically identical (unless there has been a mutation in development) and they are usually the same
sex. (On extremely rare occasions, an original XXY zygote may form monozygotic boy/girl twins by dropping the Y chromosome for
one twin and the extra X chromosome for the other. Some biologists also believe in the possibility of the egg splitting first,
then the two conceived eggs are separately semenated, resulting in identical appearance, but different sex) Monozygotic twins
generally look alike. Fine physical details such as fingerprints will differ. As they
mature, identical twins often become less alike because of lifestyle choices or external influences. Genetically speaking, the
children of identical twins are half-siblings rather than cousins. If each member of one set of identical twins marries one
member of another set of identical twins then the resulting children would be genetic full siblings. It is estimated that there
are around 10 million identical twins and triplets in the world.
The likelihood of a single fertilisation resulting in identical twins appears to be a random event, not a hereditary trait,
and is uniformly distributed in all populations around the world.[citation needed] This is in marked contrast to fraternal twinning which ranges from about 6
per thousand births in Japan (almost similar to the rate of identical twins, which is around 4-5) to 15 and more per thousand in
some parts of India[4] (and up to 24 in the US, which might
mainly be due to IVF, in vitro fertilisation). The exact cause for the splitting of a zygote or embryo is unknown.
Studies have shown that identical twins reared in different environments share similar personality traits, mannerisms, job
choices, attitudes, and interests. These findings add to the belief that many behaviors are derived from genes.[citation needed]
Identical twins have identical DNA but differing environmental influences throughout their lives
affect which genes are switched on or off. This is called epigenetic modification. A study
of 80 pairs of human twins ranging in age from 3 to 74 showed that the youngest twins have relatively few epigenetic differences.
The number of epigenetic differences between identical twins increases with age. 50-year-old twins had over three times the
epigenetic difference of 3-year-old twins. Twins who had spent their lives apart (such as those adopted by two different sets of
parents at birth) had the greatest difference.[5] However,
certain characteristics become more alike as twins age, such as IQ and personality.[6][7] This phenomenon illustrates
the influence of genetics in many aspects of human characteristics and behaviour.
A new theory (July 3, 2007) found that identical twins are formed after an embryo essentially collapses, splitting the
progenitor cells (those that contain the body's fundamental genetic material) in half. That leaves the same genetic material
divided in two on opposite sides of the embryo. Eventually, two separate fetuses develop. The research was presented at a meeting
of the European Society for Human Reproduction and Embryology in Lyon, France. Utilizing computer software to take photos every 2
minutes of 33 embryos growing in a laboratory, Dr. Dianna Payne, a visiting research fellow at the Mio Fertility Clinic in Japan,
documented for the first time the early days of twin development. Payne also discovered explanation for why in-vitro
fertilization techniques are more likely to create twins. Only about 3 pairs of twins per 1,000 deliveries occur as a result of
natural conception. But for IVF deliveries, there are nearly 21 pairs of twins for every 1,000.[8]Also, the latest twin study found that ability to listen to 2 things at once is
largely inherited. Thus, listening to someone talking, speech entering the right ear travels in large part to the left side of
the brain, where language is processed. Speech entering the left ear travels first to the right side of the brain before crossing
to the brain's language center on the left side by way of the corpus callosum, a pathway connecting the brain's right and left
hemispheres. NIDCD researchers based this finding from studies of identical and fraternal twins (national twins festival in
Twinsburg, OH, during the years 2002 through 2005). 194 same-sex pairs of twins participated in the study (138 identical pairs
and 56 fraternal pairs), representing ages 12 through 50. Researchers found a significantly higher correlation among identical
twins than fraternal twins, indicating that differences in performance for those activities had a strong genetic component. It
was found that if a trait is purely genetic, identical twins, who share the same DNA, will be alike nearly 100 percent of the
time, while fraternal twins, who share roughly half of their DNA, will be less similar. Conversely, if a trait is primarily due
to a person's environment, both identical and fraternal twins should have roughly the same degree of similarity, since most twins
grow up in the same household.[9]
Semi-identical
Monozygotic twins can develop differently, due to different genes being activated.[10]. More unusual are "Semi-identical twins". These
"half-identical twins" are hypothesized to occur when an unfertilized egg cleaves into two
identical attached ova and which are viable for fertilization. Both cloned ova are then fertilized by different sperm and the
coalesced eggs undergo further cell duplications developing as a chimeric zygote. If this chimeric blastomere then undergoes a twinning event, two embryos will be formed, each of which is chimeric for the
paternal genes (and identical for maternal genes).
This results in a set of twins with identical genes from the mother's side, but different genes from the father's side. Cells
in each fetus carry genes from either sperm, resulting in chimeras. This form had
been speculated until only recently being recorded in western medicine[11][12][13].
Demographics
A recent study found that vegan mothers are five times less likely to have twins than those
who eat animal products.[14]
From 1980–97, the number of twin births in the United States rose 52%.[15] This rise can at least partly be attributed to the increasing popularity of fertility drugs like
Clomid and procedures like in vitro
fertilization, which result in multiple births more frequently than unassisted fertilizations do. It may also be linked to
the increase of growth hormones in food.[14]
Ethnicity
About 1 in 90 human births (1.1%) results from a twin pregnancy.[16] The rate of fraternal twinning varies greatly among ethnic
groups, ranging as high as about 6% for the Yoruba or 10% for Linha Sao Pedro, a
tiny Brazilian village.[17] The widespread use of
fertility drugs causing hyperovulation (stimulated release of multiple eggs by the mother) has caused what some call an "epidemic
of multiple births". In 2001, for the first time ever in the US, the twinning rate exceeded 3% of all births. Thus, approximately 5.8% of children born in the US in
2001 were twins. Among Hausa of Nigeria and Niger the incidence of multiple births was
studied using the maternity records of 5750 Hausa women living in the savannah zone of Nigeria. There were 40 twins and 2
triplets/1000 births. Twenty six per cent of twins were monozygous. The incidence of multiple births, which was about five times
higher than that observed in any western population, was significantly lower than that of other ethnic groups, who live in the
hot and humid climate of the southern part of country. The incidence of multiple births was related to maternal age but did not
bear any association to the climate or prevalence of malaria.[18] Nevertheless, the rate of identical twins remains at about 1 in 333 across the globe,
further suggesting that pregnancies resulting in identical twins occur randomly.
Predisposing factors
The cause of monozygotic twinning is unknown.
Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:
She has a family history of dizygotic twinning, especially a mother who is a twin.
Women undergoing certain fertility treatments may have a greater chance of dizygotic multiple births. This can vary depending
on what types of fertility treatments are used. With in vitro fertilisation
(IVF), this is primarily due to the insertion of multiple embryos into the uterus. Some other treatments such as the drug
Clomid can stimulate a woman to release multiple eggs, allowing the possibility of multiples.
Many fertility treatments have no effect on the likelihood of multiple births.
There is also speculation that the West African predisposition to twinning is due to the large amount of yams in their diet because yams contain phytoestrogens, which
may stimulate the ovaries. Most likely this is nothing but a myth, as phytoestrogens would tend to suppress twinning by reducing
gonadotropin levels.
Researchers suspect[attribution needed] that as many as 1 in 8
pregnancies start out as multiples, but only a single fetus is brought to full term, because the other has died very early in the
pregnancy and not been detected or recorded. Early obstetric ultrasonography
exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes. This is known as
vanishing twin syndrome.
Conjoined twins (or "Siamese twins") are monozygotic twins whose bodies are joined together at birth. This occurs where the
single zygote of identical twins fails to separate completely, and the zygote starts to split after day 13 following
fertilization. This condition occurs in about 1 in 50,000 human pregnancies. Most conjoined twins are now evaluated for surgery
to attempt to separate them into separate functional bodies. The degree of difficulty rises if a vital organ or structure is
shared between twins, such as brain, heart or liver.
A chimera is an ordinary person or animal except that some of his or her parts actually came from his or her twin or from the
mother. A chimera may arise either from identical twin fetuses (where it would be impossible to detect), or from dizygotic
fetuses, which can be identified by chromosomal comparisons from various parts of the body. The number of cells derived from each
fetus can vary from one part of the body to another, and often leads to characteristic mosaicism skin colouration in human chimeras. A chimera may be a hermaphrodite, composed of cells from a male twin and a female twin.
Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. One fetus acts
as a parasite towards the other.
Sometimes the parasitic twin becomes an almost indistinguishable part of the other.
Partial molar twins
A very rare type of parasitic twinning is where a single viable twin is endangered when the other zygote becomes cancerous, or
molar. This means that the molar zygote's cellular division continues unchecked, resulting in a cancerous growth that overtakes
the viable fetus. Typically, this results when one twin has either triploidy or complete
paternal uniparental disomy, resulting in little or no fetus and a cancerous,
overgrown placenta, resembling a bunch of grapes.
Miscarried twin
Occasionally, a woman will suffer a miscarriage early in pregnancy, yet the pregnancy
will continue; one twin was miscarried but the other was able to be carried to term. This occurrence is similar to the
vanishing twin syndrome.
Low birth weight
Twins typically suffer from the lower birth weights and greater likelihood of
prematurity that is more commonly associated with the higher multiple pregnancies.
Throughout their lives twins tend to be smaller than singletons on average.
Identical twins who share a placenta can develop twin-to-twin transfusion syndrome. This condition means that blood from one
twin is being diverted into the other twin. One twin, the 'donor' twin, is small and anemic, the
other, the 'recipient' twin, is large and polycythemic. The lives of both twins are
endangered by this condition.
Twin studies are studies that assess identical (monozygotic) twins for medical,
genetic, or psychological characteristics to try to isolate
genetic influence from environmental influence. Twins that have been separated early
in life and raised in separate households are especially sought-after for these studies, which have been invaluable in the
exploration of human nature.
Unusual twinnings
There are some patterns of twinning that are exceedingly rare: while they have been reported to happen, they are so unusual
that most obstetricians or midwives may go their entire careers without encountering a single case.
Among fraternal twins, in rare cases, the eggs are fertilized at different times with two or more acts of sexual intercourse,
either within one menstrual cycle (superfecundation) or, even more rarely, later on in
the pregnancy (superfetation). This can lead to the possibility of a woman carrying
fraternal twins with different fathers (that is, half-siblings). This phenomenon is known as
heteropaternal superfecundation. One 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic
twins whose parents were involved in paternity suits was approximately 2.4%; see the references section, below, for more
details.
Fraternal twins from biracial couples can sometimes be mixed twins - which exhibit differing ethnic and racial features.
Among monozygotic twins, in extremely rare cases, twins have been born with opposite sexes (one male, one female). The
probability of this is so vanishingly small (only 3 documented cases[19]) that multiples having different sexes is universally accepted as a sound basis for a clinical
determination that in utero multiples are not monozygotic. When monozygotic twins are born with different sexes it is
because of chromosomal birth defects. In this case, although the twins did come from the same egg, it is incorrect to refer to
them as genetically identical, since they have different karyotypes.
The Largest Annual Gathering of Twins takes place in Twinsburg, Ohio every year on the first full weekend of August. Over
three thousand sets of twins typically attend the weekend-long event.[20]
Animal twins
Twins are common in many animal species, such as cats, sheep, ferrets and deer. The incidence of twinning among cattle is about 1-4%, and research is underway to improve the odds of twinning, which can be more profitable for
the breeder if complications can be sidestepped or managed. A species of armadillo (Dasypus novemcinctus) has identical twins (usually four babies) as its regular reproduction
and not as exceptional cases.[citation needed]
^Fraga, M.; Ballestar,
E. & Paz, M. et al. (2005), "Epigenetic differences arise during the lifetime of
monozygotic twins", Proceedings of the National Academy of
Sciences102 (30): 10413—10414
^ Segal, K. Entwined Lives: Twins and What They Tell Us About Human Behavior,
Plume Publishing, 2002
^ DeFries, J. McGuffin, P., McClearn, G., Plomin, R., Behavioural Genetics,
Worth Publishers, 4th Edition, 2000
^ Vivienne L. Souter, Melissa A. Parisi, Dale R. Nyholt, Raj P. Kapur,
Anjali K. Henders, Kent E. Opheim, Daniel F. Gunther, Michael E. Mitchell, Ian A. Glass and Grant W. Montgomery, A case of true hermaphroditism reveals an unusual mechanism of twinning, Human Genetics journal (Vol 121, No 2,
pp179-185), April 2007; ISSN 0340-6717 (Print), 1432-1203 (Online)
Nieuwint A, Van Zalen-Sprock R, Hummel P, Pals G, Van Vugt J, Van Der Harten H, Heins Y,
Madan K. (1999). "'Identical' twins with discordant karyotypes". Prenatal Diagnosis19 (1): 72-6. PMID
10073913.
Wenk RE, Houtz T, Brooks M, Chiafari FA (1992). "How frequent is heteropaternal
superfecundation?". Acta geneticae medicae et gemellologia
