Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Fraternal Siamese Twins?
Name: Amy B. C.
Status: Educator
Age: 20s
Location: N/A
Country: N/A
Date: December 2002


Question:
Is it possible for Siamese twins to be fraternal? I thought fraternal twins had their own amniotic sacs, so the fusing of the two would not happen. Identical twins can grow in their own, or individual sacs (per my cousin, who is an internal medicine doctor). If they are Siamese, they would obviously be in the same sac. I do not like to give out false information, however, so I wanted to check with an expert. What background do you have to answer this question?



Replies:
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.

vanhoeck


Amy,

My understanding of this comes as an interested reader on the subject, I do not have any special experience or training in medicine.

Conjoined twins (formerly called siamese) occur due to the incomplete division/development of one embryo/fetus apart from the other, with both originating from the same fertilized ovum. Because of this, if a set of twins is found to be conjoined, they would have to be identical twins; there would be no way for fraternal twins to fuse and share organs as is found in the conjoined form.

Thanks for using NEWTON!

Ric Rupnik



Click here to return to the Molecular Biology Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory