HOW IDENTICAL ARE IDENTICAL TWINS?

'Now, Clarice,' says Cora at last, 'you turn your lovely head to the right, so that I can see what I look like from the side'. [Mervyn Peake, 1911 – 1968].

We have all had experience with identical twins at one time or the other. Even you reading this article might be an identical twin. But are identical twins really identical? There has been a lot of research (twin studies) on the behaviour of twins, especially identical twins. It has been said that we are a product of nature and nurture, that is, every individual is a product partly of genetic influences and partly because of the environment in which he or she is raised.

According to the online dictionary, Wikipedia, Twins are two offspring resulting from the same pregnancy, usually born in close succession. They can be the same or different sex. Twins can either be monozygotic or dizygotic. Monozygotic twins are also colloquially referred to as identical twins and dizygotic twins as fraternal twins (“Twin,” 2009).

Monozygotic twins are derived when a single egg is fertilized to form a zygote which divides, as a result of mitosis, into two separate but identical embryos. Monozygotic twins are always of the same sex (except in rare cases of chromosomal defects). They also have the same traits and similar physical appearances.

We have always claimed that identical twins are identical right from their physical appearance down to their genome – DNA. A group of researchers from the University of Alabama, Leiden University Medical Centre and VU University, The Netherlands; and Uppsala University and Karolinska Institutet, Sweden recently published their findings. They studied 19 different pairs of monozygotic/identical twins. From their observation, they found differences in the CNV (Copy Number Variation) in DNA. CNVs (multiple copies of a single gene or absence of a set of coding letters in the DNA) were discovered only a few years ago.

According to Carl Bruder, PhD, and Jan Dumanski, PhD, of University of Alabama’s Department of Genetics (the study’s lead authors), “the presumption has always been that identical twins are identical down to their DNA,” “…that is mostly true, but our findings suggest that there are small, subtle differences due to CNV. Those differences may point the way to better understanding of genetic diseases when we study so-called discordant monozygotic twins...a pair of twins where one twin has a disorder and the other does not.” They also bring out the fact that while one twin might develop a particular disease, Parkinson’s for example, the other may not.

Apart from these findings, the effect of the environment must not be overlooked. While identical twins form with the same set of genes, human development is not purely genetic. There is the impact of the environment. For instance, while still in the womb, some monozygotic twins share placenta. A twin might get better share of the nutrients than the other. Also, the external world out there has its own influences.

There have been further studies carried out to verify the fact that identical twins are really identical. But the bottom line presently is that identical twins are not actually identical. And sooner or later, we would have to throw away those old biology texts.


List of References
Twin. 2009. Wikipedia 2009. Retrieved August 13, 2009, from http://en.wikipedia.org/wiki/Twin
University Of Alabama (2008, February 20). Identical Twins Not As Identical As Believed. Science Daily. Retrieved August 13, 2009, from http://www.sciencedaily.com¬ /releases/2008/02/080215121214.htm