A Genetic Study of Conjoined Twins Essay

This essay has a total of 2726 words and 15 pages.

A Genetic Study of Conjoined Twins

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. Konig. 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?

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
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