Young: Medical Genetics 
Multiple Choice QuestionsYoung: Medical Genetics
Chapter 6
Chang and Eng (1811-1874)
Photo of Chang and Eng Bunker
Conjoined twins are commonly referred to as ‘Siamese twins’ after Chang and Eng Bunker who were born in Siam, now Thailand, on 11 May 1811. They lived their entire lives fused together through a band of tissue connecting them at the lower sternum and upper abdomen. At the age of 18 years they were identified as potentially lucrative exhibits at travelling freak shows and began the first of many international tours. Ten years later they settled in North America, in North Carolina, where they lived happily until their deaths at the age of 62 years. During this period they both married and between them had a total of 22 children. When domestic relationships became strained they would alternate between their two wives’ respective homes.
As explained on page 118, twins can be either monozygotic (MZ), originating from a single conceptus which splits into two, or dizygotic (DZ) when two sperm fertilise two ova. Conjoined twinning is rare with an estimated incidence of around 1 in 100 000 births. The most common type involves fusion at the thorax (thoracophagus). A multitude of rarer forms exist including a single trunk with two heads (dicephalus) and a single head and trunk with duplication of only the lower limbs. Not surprisingly there is a high incidence of associated major malformations. In many cases survival beyond birth is not possible even with the help of expert surgical intervention.
The precise cause of conjoined twinning is not understood, with the fundamental issue of whether the basic defect involves an error of fission or fusion remaining unclear. Conjoined twins are invariably monozygotic so that the fission theory assumes only partial splitting of the primitive streak in early embryogenesis, in contrast to the complete division which results in MZ twins. Alternatively, partial fusion of two closely adjacent embryonic centres could result in conjoined twins and in some ways this would provide a more plausible explanation for the diverse phenotypes observed. These include head to head, back to back, front to front and side to side fusion. Those interested in learning more of this curious phenomenon are referred to the award winning book ‘Mutants’ by Armand Marie Leroi.
Whatever the underlying cause of their fusion, Chang and Eng once again illustrate the remarkable way in which many people are able to overcome adversity and disability, defying what might appear to others to be overwhelming odds, to live fulfilled and rewarding lives.
References
Leroi AM. (2003) Mutants. On the form, varieties and errors of the human body. Harper Perennial, London.
Collins DR (1994) Eng and Chang: the original Siamese twins. Silver Burdett Press.
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