Dear Nina
This is such an interesting subject! I have studied it recently for my book about womb twin survivors - ie. the sole survivors of a twin pregnancy.
Not only is there always an Alpha and a Beta twin (see Piontelli's work) right from the start of life, even in the womb, but that relationship stays as it is for the rest of their lives.
http://www.atypon-link.com/AAP/doi/abs/10.1375/twin.2.4.264
This is because MZ twins do differ, if only slightly, despite having identical genes, because of differing epigenetic effects on each individual.
An interesting side effect of this is when one MZ twin is left alone after the death of a co twin, they tend to assume the role of the other and become two people in one: Alpha/Beta. When the other twin dies before birth, this could be the basis of bipolar disorder - the sole survivor acts out the life of both twins to keep their sense of twin-ness alive - oscillating between an Alpha and Beta state of mind and behaviour.
Althea Hayton
Womb Twin Survivors Research Project
PO Box 396
StAlbans
Hertfordshire
AL3 6NE
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Follow my daily blog about womb twin survivors here: http://wombtwin-survivors.blogspot.com/
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The womb twin survivors research project, headed by Althea Hayton, is working directly with womb twin survivors from around the world to explore in detail the experience of losing one's co- twin before or around birth.
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