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. KÐ"¶nig. 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

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