Ask A Scientist

How are Siamese twins, in humans and animals, formed? What cells have to be mutated or deformed, if cells are deformed or mutated at all?

Asked by: Sydney Simpson
School: Chenango Valley Middle School
Grade: 9
Teacher: Michael Breed
Hobbies/Interests: Breeding Bettas and shooting videos  
Career Interest: Microbiologist or General Surgeon

Answer from Michael A. Little

Distinguished professor of Anthropology, Binghamton

Siamese twins now are known as “conjoined twins.”  They represent the tragic state where two identical twins did not fully separate from one another while they were developing at an early stage in the womb.  They are attached somewhere on the body and share some of their body parts.  Conjoined twinning occurs with both humans and other animals.     When the newly fertilized egg cell or zygote begins dividing, sometimes the cells do not stay together and begin to develop independently.  If the separation of two clusters of dividing cells is complete, then identical or monozygotic (“from one zygote”) twins will develop.  Monozygotic twins occur about once in every 250 births (0.4 %).  This is rare, but not too rare, and most of us have seen identical twins somewhere.  If the clusters of dividing cells remain partially attached, then if they survive in the womb and continue to develop, they will be born as conjoined twins.  Conjoined twins are only born once in 200,000 births (0.0005%).  Fortunately, this is an extremely rare condition because it is so harmful to the twins.  Normal twins can have a wonderful life as brothers or sisters: conjoined twins are connected much too closely to develop as independent persons.       Conjoined twinning is called a developmental abnormality, because it happens when the human embryo is forming or developing. Also, it is not believed to be caused by any genetic tendency.  So we would not refer to it as a mutation, but it is a condition where individuals can be badly deformed.     Conjoined twins can be attached anywhere on the body and there are medical names for twins who are attached in certain ways.  Some conjoined twins are attached at the body trunk and may share important organs such as the liver or the heart -- this is the most common form of conjoined twinning.  Other, more fortunate, twins are attached only by skin.  These lucky twins can be separated by surgery and then can lead normal lives.  The less fortunate conjoined twins may have to undergo very lengthy and complicated surgery, or may not be able to be separated, or the separation may require that one of the twins may have to sacrifice her or his life.  Some other information about conjoined twins is that they are always the same sex (the same as identical twins), but they are more commonly girls than boys.  This is thought to result from more conjoined twins who are boys dying in the womb before they are born, in turn leaving more girls to be born.  Only about half of all conjoined twins are born alive.     Many years ago, conjoined twins would be viewed as strange creatures and be kept with circuses or displayed as medical curiosities.  We now believe that treating unfortunate people this way is unkind and cruel.  Some physicians devote their lives to learning new surgical techniques that can help unfortunate conjoined children, and one-day scientists may learn how to prevent this terrible abnormality.  Ask a Scientist appears Wednesdays. Questions are answered by faculty at Binghamton University.  Teachers in the greater Binghamton area who wish to participate in the program are asked to write to Ask A Scientist, c/o Binghamton University, Office of Communications and Marketing, PO Box 6000, Binghamton, NY 13902-6000 or e-mail Check out the Ask a Scientist Web site at


Last Updated: 9/18/13