|MadSci Network: Genetics|
Below is the answer you are looking for about siamese twins. I hope this satisfies your inquiry.
The medical term for "Siamese twins" is conjoined twins. They can only form if they are identical. This is because of its cause. When a sperm fertilizes an egg it becomes a new cell called a zygote. Shortly thereafter, the zygote begins to divide from one cell to two, two to four, etc. This takes place in a jellylike coating called the zona pellucida. Occasionally, the zona pellucida also divides separating the cell masses, which then begin to divide also. Because these two cell masses were from the first original cell, the zygote, they are genetically identical and are called monozygotic twins. Not all monozygotic twins share the placenta and/or amniotic sac. It depends on when in development the split occurred, so they can still be monozygotic and have totally separate placentas and amniotic sacs although this is rare. Usually human females only ovulate once per month. Occasionally, more than one egg (ovum) is ovulated. Each of these may be penetrated by separate sperm and are no more alike than other siblings. They just happen to be born on the same day (usually)! They can even be from different fathers. These are called dizygotic twins. Very rarely, the cell mass divides after the point at which the cells have committed to becoming certain tissues and the cells are not "totipotent" anymore. This means that the parts must be divided between the cell masses and they must share certain structures. How many organs they share can help the doctors determine when this division occurred. So, no it is not possible for conjoined twins to be fraternal.
Information retreived from following website: http://www.newton.dep.anl.gov/webpages/askasci/mole00/mole00359.htm
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