Twins - draft copy also written and researched by Niwt (U162817)

What are twins?

A fertilised egg can split in to 2 eggs, which will produce identical children, sharing 100% of their genes. These twins are known as Identical twins, or mono-zygotic (one ovum). As these twins share exactly the same genetic make-up, they are always same sex.

Or, a woman may release 2 eggs in one menstual cycle, which produces two completely different children, just like ordinary brother or sister! These twins are called Fraternal twins, or di-zygotic (two ovums).

Monozygotic, more commonly called identical, twins, are formed when the fertilised egg, or zygtote, splits into two for no apparent reason shortly after conception. This normally happens in the first week after conception. If it happens between one or two weeks, "mirror image" twins are formed. When they stand opposite each other, it is like they are looking into a mirror, as one if left-handed, one right-handed, one completely left dominated (e.g. hair swirls and crooked teeth) and the other right. If the zygote splits after two weeks after conception, Siamese or conjoined twins may be born, because the zygote can't split properly. Conjoined twins are born attached to one another, sharing body parts and often vital organs. Depending on where they are joined, and what body parts they share, they may or may not be separated. Conjoined twins are always identical, and identical twins are always the same sex.

Dizygotic, or fraternal twins, are not identical, or though they may look similar. They are formed by two separate eggs being fertilised by two separate sperm at the same time. They only have about half their genes in common, and may be two girls, two boys, or a girl and a boy. Dizygotic twins are more common than monozygotic.

Separated Twins

Shocking as it seems, there were in fact quite a number of twins separated from each other at young ages, mainly by adoption agencies. In some cases the children knew they were twins, in others they had no idea. One pair of twins, called Beth and Amy, were separated at a very young age, and adopted by two completely different sets of parents. Beth's parents were well off, paid lots of attention to Beth, had a secure household and always spoke of Beth in a positive and loving way. Amy's parents were poor, less well educated, and ignored Amy. Amy soon developed problems such as learning difficulties and bed-wetting, apparentsly due to being the victim of bad parenting. But this was probably not so. Because despite her kind and loving parents, Beth grew up with the same problems as Amy. What this shows is that our lives may be more pre-programmed by our genes than we think.

Another interesting set of separated twins are Jack and Oskar (identical). Their parents split up when they were young, each taking one twin. Jack was brought up as a Jew by his father in Trinidad.Oskar was brought up by his Catholic mother as a Nazi in Germany. Oskar and Jack learned that they were twins, and sent a few letters to each other. When they were older, theyu arranged a meeting. They did not get on terribly well. At a later one they got on better.

People are often interested in the similarities between separated twins, especially identical ones. They may have the same of similar jobs, buy the same types of cars, have the same type of pets, same number of children, favourite foods and many other things in common. One well-known pair are the "Jim twins". James Lewis and James Springer, both nicknamed Jim, were adopted by different families at an early age. It turns out they both smoke the same type of cigarette, prefer to drink the same type of beer, both married wives of the same name, and have similar hobbies and interests, to mention just a few of their similarities. They now get on well together.

The idea of separating twins is shocking, and fortunately it is not done very much nowadays. However, separated identical twins have helped us find out a lot about how much of our lives depends on nature, and how much on nurture.

Conjoined Twins

The idea of spending the whole of your life attached to another identical being is amazing. It is, however, what happens to some people: conjoined twins.

Conjoined twins are often called Siamese twins, because they were named after the “original” conjoined twins: Chang and Eng Bunker. Chang and Eng were born in Siam on the 11th May, 1811. They were joined by a band of skin at the chest, with only a thin strip of liver inside it. Other than that, both were fully-formed, with two legs and arms. Chang and Eng made their living from their twinness, as “freak” shows. They married sisters, and fathered 22 children between them. Chang died first, from a stroke, and Eng bled into his dead brother’s body and died a few hours later.

Nowadays Chang and Eng could have been separated without any problem. Lots of pairs of Siamese twins are separated today, with a reasonably high success rate. Some who share vital organs are less easy to separate, and there is a high chance that one or both of them will die during or after the operation. Recently there was a case that made newspaper headlines about Siamese twins brought to England for surgery. The Maltese parents, who were Catholic, did not want their girls separated as they claimed it was “against the will of God”. The stronger twin was more or less keeping the weaker twin alive, but her strength to do this was draining and without the operation both would have died within weeks. With the operation, the stronger twin (called ‘Jodie’) would be likely to survive, and while the success rate for ‘Mary’ was lower, it was possible she would have survived too. After a long battle in the courts, it was decided that the operation would go ahead against the parents’ will. ‘Jodie’ survived the operation. ‘Mary’ didn’t.

Other Siamese twins look like Zaphod Beeblebrox, with one body and two heads, and some have two bodies and one (or one and a half) head/s, like a pair in America called Lori and Riba. They actually share large brain portions!

How have twins been used in scientific research?

Scientists have found the study of twins beneficial to their work in genetics and environment. They are interested in the effects that genetics have in our development, and also the effects that the environment plays in our development. Twins who are identical, share identical genes, but not always the same environment.

The hypothesis is that twins who are reared separately, yet have similar personality traits and medical diagnoses, are influenced by their genes (and hence, are we all). However, if these similarities are not present in those identical twins who are raised apart, then the environment plays a larger role in influencing our development. This has been called the nature-nuture argument. More recently, scientists are beginning to realise that it is a mixture of genetics and environment that influences our development.

Twins have also been used for research in the area of intelligence testing. Intelligence quotient (IQ) tests claim to measure a person's mental aptitude without language or culture factors being involved. It was found that when testing twins, their IQ's had a higher correlation than any other group of people (siblings, parents, or half-siblings. The monozygotic twins, identical twins, scored more than di-zygotic, giving weight to the argument that intelligence is influenced by genetic biology. However, in most cases these twins were reared together, so social influences could also be at work. But, the counter-argument against this, is that mono-zygotic twins reared apart still have a higher correlation of IQ score, than ordinary siblings. But it is still noted that identical twins reared together have a higher correlation, than those idents raised apart, again agreeing that there is a mixture of genetics and environment at work.

Female mono-zygotic (MZ) twin pairs

Male MZ pairs

Female di-zygotic (DZ) twin pairs

Male DZ pairs

Female sibling pairs

Male sibling pairs

Critics of this research state that twins being used in research is rarely for the benefit of twins, instead results being used to generalise to the rest of the population. Do twins make good subjects? As they make up a relatively small percentage of the population, they are not really a good cross-section of society. Should we use their data to generalise to the rest of the population? Until scientists find an alternative method of research (genetic cloning) then the jury is out on this one.

Experiences of being a twin

There are some reports from Twins, especially identical twins, that they can share experiences, even though they are separated at that time. These experiences normally involve something traumatic, or highly relevant.

Twin Telepathy?

Many sets of twins claim to have been involved in some sort of Extra Sensory Perception, or ESP. Some twins say they know when their twin is angry or upset, even when they two twins are in different countries. There have been various cases of one twin crying whilst the other is being hit, one twin feeling labour pains whilst the other is giving birth, and one twin feeling heart pain whilst the other's having a heart operation. In all these cases the twins were far apart from each other.

Other twins have gone shopping at different times and bought exactly the same clothes as each other, with no prior communication between them. Some twins have been known to call each other up at exactly the same time. But the real twin telepathy test haven't shown any evidence that twin telepathy exists, so we can never be sure. Probably identical twins just think in similar ways, leading them to do similar things, because their genes are the same.

Handedness in Twins

There is a myth that every left-hander once had a twin who died in the womb. Although this is probably not correct, strangely enough about twice as many twins as single-born people are left-handed. There are various possible explanations for this. One is because there is one left-handed twin with every pair of mirror image twins. Another idea involves one twin copying the other. When the twins are just learning how to write, they might sit oppostie each other. If one twin picks up a pen in their right hand, and writes like that, the other might pick it up in their left hand, mirroring the first.

Left-handedness is associated with birth difficulties, and the birth of twins is often difficult.