Twins are remarkably interesting but widely misunderstood. Twins result from an unusual pregnancy in which the mother gives birth to more than one child. The number of twins or multiples, their genetic similarities, and their similarity in appearance can all vary widely, as can the cause and mechanism for the twinning. Scientific and sociological interest in twins is intense because they help us learn about genetics, embryology, human development, and the effects of environment vs. heredity.
Types of Twins
Most people are familiar with fraternal twins, identical twins, triplets, quadruplets, and conjoined twins. But there are many other types of twins and many variations on the former.
Fraternal twins are simply two normal siblings who happen to be born at the same time. They share few additional similarities compared to ordinary siblings (other than the same birthday, and some marginal added commonality due to coincidental development). Fraternal twins can be same sex or different and can be as similar or different as any brothers and sisters. On average they share exactly half of the DNA similarities of each other and all other siblings, though like other siblings, in extraordinarily rare cases they can still actually be identical (more about that later). Fraternal twins are actually properly called dizygotic (two eggs) or DZ because they occur when the mother releases two separate eggs and both are fertilized by different sperm from the father.
Identical twins occur when the egg splits after fertilization. The proper term is monozygotic or MZ. Both twins develop from the same egg and sperm and therefore are identical. It’s important to realize that identical twins are not actually 100% identical but we use that term because they usually look very similar and have almost identical DNA and genes.
Higher order multiples such as triplets and quadruplets are more rare forms of twins. They are most commonly fraternal due to the fertilization of multiple eggs. However, higher order multiples can be any combination of dizygotic (DZ) and monozygotic (MZ) twins.
Conjoined twins occur when the egg splits after development is further along and the (usually) two twins do not fully separate. Conjoined or colloquially siamese twins can be joined just about anywhere and their survival prospects and outlook depends heavily on how much they share. Conjoined twins are thus alway MZ and same sex,
Mirror Image twins are identical twins where the egg splits after seven days but before the point that conjoining would result. About 25% of identical twins are mirror image. Mirror image typically means that one twin is superficially mirrored in the other. One may be right-handed while the other left-handed, their hair whorls may be opposite, and they may have markings or moles on opposite sides. In very rare cases, mirror image twins can be entirely reflections of each other right down to the arrangement of organs. For example, it’s possible for a mirror image twin to have their heart on the right side, their liver on the left side, and their appendix on the left side. (imagine the difficulty in doing a heart transplant!)
Polar body twins or half-identical twins occur when the egg splits before fertilization and each half (the egg itself and it’s polar body) is fertilized by a different sperm and develops separately. In such cases, the twins are halfway between fraternal and identical and share on average 75% of their DNA in common. There is considerable controversy over polar body twinning and its frequency. No easy tests are yet available to determine it. Kate and Ashley Olsen are thought by some to be polar body twins.
Twins of two is a term that means half-fraternal twins. This occurs when the mother releases two eggs and each is fertilized by a different partner’s sperm. The twins happen to both be viable and are born at the same time. Such twins have 25% identical DNA just like half-siblings.
Vanishing twin. Some conjoined twins, and most identical twins for that matter never reach birth to develop as separate individuals. The second twin is more often not viable on its own due to some defect or disadvantage in development. Often this second twin is never known to have existed. Only one twin is born normally and the other vanishes (doesn’t mature).
Parasitic twin. Rather than vanishing completely an underdeveloped conjoined twin can occasionally even develop within the other twin and survive parasitically for years.
Identical or not?
Parents of twins usually want to know whether they are identical or not. Currently there’s not a foolproof way to tell, but the only highly reliable way is a DNA test (99.9% conclusive). One of the best ways to tell is by taking a simple questionaire about visible similarities. Twins that look remarkably alike are usually MZ twins, and obviously those that do not are DZ. Many new mothers are told whether their twins are DZ or MZ based on whether they share a chorionic sac or placenta. However, that method is wrong at least 25% of the time because MZ twins can sometimes have separate sacs or placentas. The opposite is much rarer but can also occur. Half-identical twins are even harder to determine. It’s largely still a question of speculation because reliable DNA test haven’t been established to determine this type of twin. Since identical twins are never actually entirely identical, sometimes the mother will assume they are DZ when they are not. And conversely, because DZ twins can look remarkably alike, they will often be mistaken for MZ twins.
Perhaps the most interesting thing about twins is that they can have a wider variety of genetic similarity than most siblings. For comparison, each person gets exactly half their genes from their mother and half from their father. This means that you are exactly half identical genetically to both your parents. It also means that you are very close to half-identical to your non-twin siblings – it’s very likely that this is very very close to half identical, but it’s a random process so it’s possible (perhaps so rare that it has not even happened) that you could be anywhere from 0% to 100% identical to a sibling! If you have half brothers and sisters, you are 25% identical to those (like your cousins on average), but it can range from 0% to 50%. Interestingly, when identical twins have babies, those children are genetically siblings (50% identical) rather than being genetically cousins (25% identical).
Fraternal twins are exactly the same case as ordinary non-twin sibblings (50%). Identical MZ twins are 100% similar genetically. Whereas half-identical twins are 75% genetically identical. And twins of two are 25% genetically identical. Of these, the really interesting cases are MZ and polar body (half identical) twins because they exhibit genetic similarities that do not occur in other ways in the general population (disregarding the random possibility that it will occur between ordinary siblings on astronomically rare occasions). Scientists use MZ twins to test theories about what is determined genetically, so called nature vs nurture. The idea is that if two people with the exact same genes react or develop differently, the thing being study much have a nuture or environmental component. It’s not that simple as we’ll see below, but it’s still fascinating to see what differences MZ twins develop and what similarities they maintain.
What’s identical in MZ twins?
People often ask whether identical twins have the same fingerprints, eye color, sex, hair color, or personalities. The answer varies. For traits that are determined genetically, the answer is usually yes (sex, eye color, hair color, etc.) For traits that are phenotypic (determined by development) such as fingerprints and personality, the answer is no. Even with genetic traits such as handedness (most often), there is the case of mirror image effects that can cause a difference.
Are identical twins actually 100% DNA identical?
In general the answer is yes. At least their DNA is as similar as any two people’s can be (other than clones which have not yet been raised). MZ twins were conceived from a single zygote (fertilized egg) of which half of its DNA came from the father and half from the mother. When the eg split, both halves of the division had identical DNA and so the twins develop with this identically matching DNA. One clear exception is the mitochondrial DNA which is always different in MZ twins and can also cause some biological differences.
For many years MZ twins have been used to determine whether certain conditions, diseases, and traits were genetic (nature) or environmental (nurture). The assumption being obvious that if both twins had the condition, it was most likely due to their common genes. If only one had the condition it was due to their environment or upbringing, or phenotypic differences (those caused by different embryology and development such as fingerprints and many other traits). But once certain conditions and traits were proven to be genetic, scientists found that not all identical twins had similar conditions. Recent research has shown that there are a variety of reasons why MZ twins’ DNA may not match exactly. The different processes and reasons are too complicated for me to fully understand much less summarize here, but it’s been shown conclusively that MZ twins’ DNA does not always match exactly, and may never match exactly. Current DNA tests compare key samples of DNA rather than testing the whole DNA strand which is much to long and difficult, so the direct evidence is still sketchy about just how much DNA MZ twins share.
- Twins do sometimes develop secret languages between one another.
- Twins cannot read each other’s minds, but they often have amazing abilities to understand the other or know how they are feeling.
- Female identical twins are more common than male identical twins.
- Twins have about twice the likelihood of being left-handed than non-twins.
- Twins are almost always born prematurely, moreso the higher the number of multiples.
- Twins can be born minutes, days, or even months apart. Typically true twins are born minutes apart and its not significant which is born first.
- The idea of an evil twin and a good twin is a myth. Though often one identical twin can be larger, stronger, or more dominant than the other based on how they developed.
- Identical twins make excellent organ donors for one another.
- It’s exceedingly rare but still possible (3 known cases) for identical twins to be different sexes.
- Cats, sheep, and ferret have very high incidences of twins
- Eating Yams can increase your chances of having twins
- Older mothers, african mothers, and mothers that have had twins are more likely to have twins
- A predisposition to have fraternal twins may be mildly hereditary, but identical twins are simply luck. It’s a myth that twins skip a generation.
- Recent usage of fertility drugs has increased the occurence of twins, especially higher order multiples.
- Half of all MZ twins sharing the same chorionic sac die due to umbilical chord entanglement.
- Identical twins occur in 1 out of 150 births worldwide.
- There’s a town in Brazil that has a 10% twinning rate.
- Identical twins have different fingerprints but similar fingerprint patterns.
- Identical twins have identical gender, eye color, hair color, and blood types.
- Identical twins live longer than fraternal twins