Saturday, 1 September 2007

Twins: Identical, Mirror Images, Fraternal and Chimeras

Cloning is not a human invention; nature has been creating clones for millions of years, among all organisms including humans. Nature’s clones, identical twins, are born in approximately 1 / 1000 births. Identical twins come in two varieties: identical and mirror images. Both share 100% of their DNA and but in mirror image twins, small differences are ‘reflected’. Examples include skin variations such moles, dental patterns, hairlines and handedness.

The development of a truly identical twins versus mirror image twins comes down to timing. A single sperm will fertilize a single egg and begins development by splitting into more cells. If this group of cells, now called blastocyst splits into two separate parts in the first 9-12 days, identical twins will be born. But if the split occurs after that, they will be mirror-images of each other.

Fraternal twins are an entirely different matter. Fraternal twins are no more identical than any other sibling pair and are the result of two separate sperm fertilizing two separate eggs. This is can occur naturally, the result of the mother releasing more than one egg at ovulation. It may also be the result of medical intervention as many women take fertility drugs to improve their chances of conception. There is also a hereditary link as the incidence of fraternal twins do occur more often within a family.

Many people have seen the popular American television show CSI (Crime Scene Investigators) and may recall the episode with the Chimera, a man who had two sets of DNA. This phenomenon occurs when the blastocysts of developing fraternal twins fuse, resulting in a single individual with two sets of DNA. This condition usually results in a fully functional individual and is not detected unless a clear abnormality prompts testing. Though it has been considered a rare condition, it is found to be more common than originally thought in a variety of animals, including humans. And the condition is more common among children conceived through in vitro fertilization than naturally.

By the way, you can read about the amazing birth of identical quadruplets from my hometown, Calgary, Canada here.

6 comments:

Mario Pineda-Krch said...

Of course chimeras are much more common than what people think. I have been preaching this for years. That's why I wrote two papers about it (Pineda-Krch & Lehtila. 2004. both in J. Evol. Biol. vol. 17). Despite this it is still an uncomfortable idea among many biologists. Never heard of CSI though - sounds like an enlightened bunch.

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archaeozoo said...

How interesting. I'd never heard of the chimeras before.

Anonymous said...

I had never heard of a Chimera until I was called one. I was told that I most probably started out as a mirror twin, but that the egg didn't completely split. My left side is much smaller and weaker than the right side, but internally the organs on the left side are healther and larger than the right. I have two extra ribs and I had a tooth in my sinus. These are just a few of the abnormalities that have been found over the years.

AJC said...

Have you ever heard of Chimeras where both twins are still alive? I'm a tech in a lab and it was discovered that a female twin had about 80% XY male cells in her blood. A skin sample was then taken and the majority of the cells in her skin was female with a small percentage of male cells. Her male twin was then tested and he also had 80% male cells in his blood and 20% XX, female cells. This would make them both living chimeras suggesting that they touched and exchanged cells when they were probably only 8 stem cells along. A co-worker has a theory that it happened later on and was a result of twin to twin blood transfusion. Is this possible?

Anonymous said...

Very Interesting!
Thank You!