John37309 said:
Monique and Bobze,
Thanks for your help.
I was very unprepared when i posted this message. Its as simple as this; I need to spend some time reading up on this topic if i want to understand it. So you have given me some very helpful tips to get started.
Thanks guys,
John.
Many of the differences between “identical” twins originate in the placental environment. Although the twins are genetically the same, the zygotes become implanted in different places in the uterus. Even if the twins share a placenta, they have to be attached to the placenta in different positions. Therefore, the environment of the twin embryos has to start out slightly different.
The environmental differences in different parts of the uterine wall may be small. However, they get amplified by the competition between the embryo for the resources supplied by the mother. One twin gets slightly “short changed” by the mother. The twin that starts out getting slightly more resources can draw out even more resources from the mother. Therefore, one twin may be slightly undernourished or underoxygenated than the other early in development. Therefore, one develops differently from the other. The developmental differences at birth can persist throughout their lives.
There have been many studies of the effects of embryonic competition on the development of twins. Here are a few links on the subject.
https://sfari.org/news-and-opinion/blog/2012/twin-differences
“The researchers found that epigenetic profiles vary significantly among both types of twins. They speculate that the differences arise partly randomly and partly from environmental factors, such as differences in blood supply, exposure to infection and the weight of each twin’s placenta.
In an unexpected twist, identical twins who shared a placenta show greater differences between their epigenetic profiles than identical twins who did not, suggesting that competition for resources may play a role.”
Identical triplets and identical quadruplets vary even more than identical twins because of this competition.
http://www.encyclopedia.com/topic/Multiple_pregnancy.aspx
“Of course, pregnancies with more than two babies have occurred throughout history. However, once the number of babies reaches three, overexpansion of a woman's uterus begins to cause difficulties. The implantation of several embryos and placentas in the endometrium of the uterus results in a competition for space and inevitably some implant in an area without good circulation. During a pregnancy, it is essential that the uterus be well perfused to sustain the fetus with nutrients and oxygen. A lack of oxygen can cause central nervous system damage in the fetuses that implanted in a less than desirable area. Since the human female was not made to carry an indefinite number of fetuses, multiple gestations can have many of the following complications:”
The competition for placental resources can result in the ultimate difference between identical twins in humans and in animals. One twin can survive and the other die before birth. This study showed that this is quite common among horses.
http://pregnancytreatment.blogspot.com/2010_03_01_archive.html
“Multiple ovulations tend to be more common in barren and maiden mares then in lactating mares… Most twin pregnancies will terminate, either by early foetal reabsorption or the birth of small retarded foals.
Placental competition leads to insufficiency and usually results in abortion.”
I don’t think this is as common in humans as in horses, but it does happen in humans. Elvis and Jesse come to mind.
The amplification of differences due to competition is a special example of what physicists call spontaneous symmetry breaking. Spontaneous symmetry breaking describes any physical process where an initial difference between different physical states is amplified by a nonlinear interaction. The result is that the final states are far different even though the initial states had no apparent difference.
http://en.wikipedia.org/wiki/Spontaneous_symmetry_breaking
“Spontaneous symmetry breaking is a mode of realization of symmetry breaking in a physical system, where the underlying laws are invariant under a symmetry transformation, but the system as a whole changes under such transformations, in contrast to explicit symmetry breaking. It is a spontaneous process by which a system in a symmetrical state ends up in an asymmetrical state. It thus describes systems where the equations of motion or the Lagrangian obey certain symmetries, but the lowest energy solutions do not exhibit that symmetry.
Consider the bottom of an empty wine bottle, a symmetrical upward dome with a gutter for sediment. If a ball is placed at the peak of the dome, the situation is symmetrical with respect to rotating the wine bottle. But the ball may spontaneously break this symmetry and roll into the gutter, a point of lowest energy. The bottle and the ball retain their symmetry, but the system does not.”
In the case of identical twins, the “underlying laws” are the genome of the twins. The genome of the twins are invariant to exchange of the individuals. The system consisting of the two twins is almost invariant to individual exchange, but initially there is a slight asymmetry due to uterine location. The competition between the twins is an interaction that amplifies the initial difference so that the “identical” twins can be noticeably different even from birth.
Please note that competition between twins can continue after birth. Therefore, the twins can become more different as they grow up. This starts to be a free choice. The competition amplifies the difference whether or not the initial difference is prenatal.
I once saw a Jeffrey Springer TV program where three sets of identical twins were interviewed. Although I generally avoid such sensationalist programs, the topic interested me enough to make an exception.
One set of twins made a decision early in life to try to be as different from each other as they can possibly be. The other two sets of twins made a decision to be as similar to each other as they can possibly be. The twins that made the decision to be different dressed differently, went to different vacation spots, and tried to make their experiences as different as possible. The two sets of twins that decided to be as similar as possible married each other!
