A Genetic Study of Conjoined Twins

A Genetic Study of Conjoined Twins

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1.0 Introduction

I have always been fascinated by conjoined twins and have always had questions about them like; what do the Siamese have to do with conjoined twins? Why does this form of twin happen? What, if any genes cause this? What types of Conjoined twins are there? How does the environment affect, if at all, the biological families' gene pool? In my research in efforts to prepare this paper, I found the answers to this question and many more. This term paper will cover the types of conjoined twins, the biological occurrence that causes conjoined twins, a look into some of the genetic and environmental causes of conjoined twins, the types of conjoined twins and the genetic and social impact of conjoined twins.
1.1     Siamese - or - Conjoined Twins

Let's answer the first question right off the bat. The terms Siamese Twins and Conjoined Twins are synonymous, 1 The term Siamese twins comes from the most famous of conjoined male twins, Chang and Eng Bunker, born in Siam of Chinese parents in 1811. The Bunker Twins were exhibited in Barnum's circus for many years. While they were never separated, they each married and were successful businessman and ranchers in Wilkes County, North Carolina. Chang and Eng were attached by a five-inch connecting ligament near their breastbones. Although the Bunker Twins were connected each of them and their wives, sisters Sallie and Adelaide Yates, lived fairly private lives when they weren't touring the world to earn income. The twins died within 2 hours of each other in 1874. After their deaths it was determined they could have been successfully separated, a medical option that was never offered to Eng and Chang during their lives.
It was Eng and Chang's fame that helped coin the phrase 'Siamese Twins'. It should be noted that they were not the first pair of conjoined twins recorded in medical annals. There were approximately one hundred pairs of conjoined twins known by the time of their 1811 births. This fact supported the King of Siam's decision to reverse an early death sentence on the brothers. Fact of the matter is, conjoined twins were recorded as early as 945 in Armenia with the first pair of successfully separated twins occurring in 1689 by German physician G. König. The term Siamese was later replaced with the more scientifically and sensitively correct and precise term conjoined.

1.2     What Process Happens or Doesn't Happen that Causes Conjoined Twins?

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In layman's terms, the zygote doesn't totally separate during development. Conjoined twins is a very rare form of identical twins that occurs approximately in one out of every 75,000 to 100,000 births or 1 in 200 deliveries of identical twins. Conjoined twins originate from a single fertilized egg that is developed monozygotically. Therefore the twins are always identical and same-sex twins.

Biologically speaking, what happens is the developing embryo starts to split into identical twins within the first two weeks after conception but then stops, for various reasons most unknown to man, before completion. This halt in process leaves a partially separated egg, which continues to mature into a conjoined fetus.

Specifically, embryology, the study of embryos, states that only monozygotic twins can be conjoined. Statistics show that monozygotic or identical twins account for thirty percent of all twins. The genetic process for the conjoined twin begins four days after the diploid cell formed by union of two gametes is fertilized by a sperm, the trophoblast (chorion) changes. If the split occurs before this time the monozygotic twins will implant as separate blastocysts each with their own chorion and amnion. Twenty five percent of monozygotic twins are dichorionic. All dichorionic twins are diamniotic.

Eight days after fertilization the amnion differentiates. If the split occurs between the 4th and 8th days, then the twins will share the same chorion but have separate amnions. Monochorionic diamniotic is the most common form at monozygotic twins, accounting for seventy-five percent of monozygotic twins.
If a split occurs after the 8th day and before the 13th day, then twins will share the same chorion and amnion. This is a very rare condition and accounts for up to two percent of monozygotic twins.
The embryonic disk starts to differentiate on the 13th day. If the split occurs after day 13, then the twins will share body parts in addition to sharing their chorion and amnion.
1.3      How often Does this Occur and What are Other Conjoined Twin Statistics?

Medical analyses have recorded conjoined twins as early 945. There are statistics that assist scientist and doctors with the occurrence of conjoined twins. However, no one can specifically state with full confidence why the cell doesn't completely divide - or why the cell division just stops. There are several facts that may link environmental elements to the cause for conjoined twins. Specially, the birth of conjoined twins is more likely to occur in India or Africa as opposed to China and the United States. Why? No one seems to know for sure. However studies of geographically influenced diets, DNA and other environmental similarities and differences are being studied. Scientist believe that diets and other environmental causes or situations may be directly linked to the condition(s), which are responsible for the failure of twins to separate after the 13th day after fertilization.
Unfortunately, or maybe fortunately, the occurrence of conjoined twins is not frequent enough to develop solid test data. Since conjoined twins occur approximately every 40,000 births but only once in every 200,000 live births environmental and other test data are difficult to capture.

Other statistics seem to puzzle scientist and doctors about conjoined twins. Such as, conjoined twins are more often female than male this is a 3:1 ratio, even though monozygotic twins are more frequently male than female. 1/50,000 to 1/100,000 births are conjoined twins but forty percent are still born and seventy-five percent die within 24 hours. The overall survival rate of conjoined twins is approximately five to 25%.

1.4     Is there other Conjoined Multiple Births?

There are no known or documented cases of conjoined births other than twins. However there is one documented case of triplet births that featured conjoined twins, most recently Nida and Hira Jamal of Pakistan. Although the two Craniopagus girls were successfully separated, tragically Nira's heart wasn't strong enough after the separation and she died shortly after the operation.

1.5     What are the Classifications or Forms of Conjoined Twins?

Conjoined twins are generally classified by the point at which they are joined. The term used is developed based upon the Greek word pagus, which means 4."That which is fixed.") Hence, the suffix-pagus is used meaning fastened. Over the last century, scientist and doctors have termed and identified more than three dozen separate types of conjoined twins.
While there are dozens of types of conjoined twins, doctors generally divide the types into the more common variations. All of these forms can be more broadly categorized as displaying either equal and symmetrical forms or unequal and possible asymmetrical forms.
The following basic classifications can be combined to more closely define individual cases.

1.5.1     Cephalopagus
This form of conjoined twin has an anterior (early in development) union of the upper half of the body with two faces on opposite sides of a conjoined head. This type of Conjoined condition is very rare. Sometimes this type of twin shares one heart.
4. Conjoined Twins Website - http://www.conjoined-twins.i-p.com
1.5.2     Craniopagus           
This is when the twins are joined at the cranium (the top of the head or skull). This form of conjoined twin occurs very rarely about 2% of all conjoined twin cases. This class of conjoined twin is a very difficult type of twin to separate. An example of Craniopagus Twins is provided below:
5. Conjoined Twins Website - http://www.conjoined-twins.i-p.com

1.5.3     Thoraopagus          
This is the most common form of conjoined twins. This form of twin occurs in between 35-40% of all conjoined twin cases. The Thoraopagus twins share part of the chest wall, possibly including sharing the heart. Note the picture below:

6. Types of Conjoined Twins - http://zygote.swarthmore.edu/cleave4a.html

1.5.4     Pygopagus     
This form of conjoined twins are more than likely positioned back-to-back and usually have a posterior connection at the rear-end. This form of conjoined twin occurs in almost twenty percent of documented cases. This form of conjunction always involves the umbilicus. This type of union is also known as Illeopagus. The famous Millie and Christine McCoy twins born into slavery are examples of this form of conjoined twin.
7. Conjoined Twins Website - http://www.conjoined-twins.i-p.com
1.5.5     Ischiopagus     
This form of conjoined twin is joined at the pelvic. Approximately six percent of all conjoined twins have this condition. In scientific terms the twins are joined by the coccyx (lowest part of the backbone) and the sacrum (backbone immediately above the coccyx).     

8. Conjoined Twins Website - http://www.conjoined-twins.i-p.com

1.5.6 Omphalopagus

This form of twins is united from the waist to the lower breastbone. This form accounts for approximately thirty-four percent of all conjoined cases.
9. Conjoined Twins Website - http://www.conjoined-twins.i-p.com

1.5.7     Crainiothoracopagus
This form is very rare. It is when there is a union of head and chest. The twin shares one brain, and the hearts and gastrointestinal tracts are fused. The other term for this form is known as epholothoracopagus. The picture below provides a graphic specimen of this form of conjoined twin.
9. Conjoined Twins Website - http://www.conjoined-twins.i-p.com
1.5.8     Dicephalus
This conjoined twin form is defined as one body with two separate heads and necks. Abigail and Brittany Hensel of the United States are an example of this very rare type of conjoined twin, please note a diagram of their body below:
10. Conjoined Twins Website - http://www.conjoined-twins.i-p.com

1.6     The Myth the Monster
In addition to the basic types of conjoined twins, there are other types of conjoined imperfect twins if you will. These types have been unfairly and unjustly termed as "freaks and even worse "monster". Provided below are a couple of examples of these conjoined twin forms.
1.6.1     Monocephalus Diprosopus
This form of conjoined twins is caused by incomplete duplication of primitive streak leading to imperfect formation of each baby and conjoining.     Duplication of the cranial portion only:
11. Monochorionic Twinning: Sonographic Assessment. Filly RA, Goldstein, RB and Callen PW. AJR 154:459-469, March 1990. Parasitic Twins -
This form is when asymmetrical conjoined twins are developed. One twin is smaller and less formed than the other and is usually dependent upon the other twin.     Fetus in Fetu-
This form is when one twin, an imperfect fetus is contained completely or partially within the body of its sibling Hemiacardius
This incomplete form of conjoined twin has some part of the less formed baby missing but the core structures remains Holoacardius
The more and important part of the less formed baby is missing i.e., No head - Holoacardius acephalus, No trunk - Holoacardius acormus.

1.7     Conjoined Twins Yesterday and Today
Throughout history, conjoined twins have been represented in as monsters, omen, myths and legends. The Greek and Roman god Janus had two faces, an old and young face. Centaurs, was a combination horse and man, which may have been inspired by parapagus twins who often have four legs. Then there is the common heraldic symbol, the Double-Headed Eagle", which is known throughout Central Europe.

One of the earliest documented cases of conjoined twins is Mary and Eliza Chulkhurst, also known as the Biddenden Maids. Born in 1100, the sisters lived for 34 years in Biddenden, County of Kent, England. Mary and Eliza, though often depicted as joined at the hip and shoulders, were likely pygopagus twins who were joined at the buttocks and lower backs. After the death of one sister, doctors hoped to save the life of the other by separating them surgically. The surviving twin refused, declaring, "As we came together, we will go together." She died several hours later.

With today's technology, conjoined twins are being detected in the womb and in many cases successfully being separated after birth. Of course, some types of conjoined twins, separation is much easier to separate. Then there are other rarer forms of conjoined twins where separation is complicated and costly. And the decision could mean life for one and death for the other. These procedures lead to difficult ethical and moral decisions of separation surgery, especially if the twins share internal. According to the book medical records, there have been approximately 200 attempted surgical separations of conjoined twins, with about ninety-percent of these occurring after 1950. About 75% of these separation procedures since 1950 have resulted in one or both of the twins surviving.
In recent years, famous conjoined twins, Angela and Amy Lakeberg born in Indiana on June 29, 1993, the twins were joined at the chest and shared a heart and liver. They could not survive together as the heart was not strong enough. Given that it was decided to separate them, ultimately sacrificing one for the other. Angela, the stronger of the two, was chosen to be the survivor. Angela never got the chance t go home. Ten months after the separation surgeries, Angela died in the hospital of pneumonia.

This death raised powerful questions about the ethical and economic costs of separation. For some the financial expense was too expensive, costing in excess of $1M. Others (the medical community) believed the surgeries were worth the pain and suffering. They believe that the skills and knowledge acquired from surgeries like the one that separated Angela and Amy would aid in future attempts. The challenge for many observers lies in the conflict between the desire to help medically fragile conjoined twins like Angela and Amy, and the need to justify the emotional and economic costs of their care. Or simply are we playing God?
There are quite a few conjoined twins in today, surgical separations occur more frequently and with greater success than ever before. Conjoined twins remain a topic of scientific speculation, public interest, and an image of two minds in the same body.


I am very glad to have had this opportunity and challenge to learn more about conjoined twins. After my research, viewing the pictures, reading the life stories, I have a newfound respect and admiration for conjoined twins. Monsters? I think not, beautiful and uniquely challenged gifts to mankind? Absolutely

- End -


1. Body Doubles: Siamese Twins in Fact and Fiction", an exhibit constructed by Laura E. Beardsley at the Mütter Museum, Philadelphia, PA, Spring 1995.

2. America's "United Siamese Brothers": Chang and Eng and nineteenth century ideologies of democracy and domesticity. In Reading Monsters/Reading Culture (ed. J. J. Cohen), University of Minnesota Press, NY. Pingree, A. 1996.

3. Duplicate twins and monsters. Wilder HH. Am J Anat 1964 ; 3 : 387

4. Conjoined twins. In : Votteter TP. Welch KJ, Randolph JG, Ravitch MM, O’Neill JA Jr., Rowe MI, eds. Pediatric Surgery, 4th edition. YearBook Medical Publishers. 1986 : 771-9

5. Conjoined Twins Website: http://www.conjoined-twins.i-p.com or http://go.to/twins

6. Conjoined Twins Prenatal Diagnosis and Assessment of Associated Malformations. Barth RA, Filly RA, Goldberg JD, et alRadiology 1990;177:201-207.

7. Monochorionic Twinning: Sonographic Assessment. Filly RA, Goldstein, RB and Callen PW. AJR 154:459-469, March 1990.

8. A Glossary of Genetic Terms - http://www.weihenstephan.de/~schlind/genglos.html

9. The Virtual Embryo - http://www.ucalgary.ca/UofC/eduweb/virtualembryo

10. Virtual Zygote Website - http://sdb.bio.purdue.edu/Other/VL_DB.html

11. Glossary of Terms - The University of Edinburgh, Edinburgh School of Biology, Biology Teaching Organisation, website: http://helios.bto.ed.ac.uk/bto/biology/index.htm

12. Conjoined twins," World Book Online Americas Edition", Melvin V. Gerbie, http://www.aolsvc.worldbook.aol.com May 27, 2001.

13. Fictional characters in Twin Falls Idaho

14. Types of Conjoined Twins - http://zygote.swarthmore.edu/cleave4a.html

15. Conjoined Twins Website - http://www.conjoined-twins.i-p.com

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