Thursday, April 21, 2011

The twin within (4) A chimera

What is a chimera? 

A person who is a chimera is made up of cells from different people. So some of their cells have the DNA of one person and the other cells have the DNA of another person. How do you do that?

Imagine that inside of a womb, instead of there being only one egg, there are actually two. If both of these eggs get fertilized by two different sperm cells, you get fraternal twins. Fraternal twins are no more related than any brother or sister.

Now, imagine that instead of developing separately, these fertilized eggs actually fuse. Then only one baby would develop. This baby would have cells from not one, but two different
zygotes.

Remember, every zygote carries its own unique set of DNA. So the baby would have two different sets of DNA-this baby would be a
chimera. 


IF you have ever had a transplant you are a chimera, because you have cells of different  genetic makeup sharing your body.  Even a blood transfusion will makes you a chimera for a few weeks, until the blood cells die.

Does the presence of cells of another human in your body make any difference? I hear you ask. Well, it does.

Microchimerism, that is the presence of a very few cells from another human remaining within the body, is common  in women after pregnancy.  ( see article here)
Autoimmune diseases are thought of as disorders in which a body's cells inexplicably attack its own normal tissues.


The physical consequences are that your immune system will attack your own body - at least the individual cells of your body that are "foreign" because of the differing DNA.


The psychological effects are more subtle: a sense of being more than one person, as indeed you are.  For a woman after pregnancy, a sense of being forever bonded to her children, living and dead, as if they were still within her. As a mother myself,  I feel a strong blood bond with my children, now both far away, and even the one I miscarried at 3 months of pregnancy 37 years ago. Perhaps it is possible to feel a sense of bonding and connection to the cells of another person within your body.  If the DNA is not so very different ( a fraternal twin sibling does share half your DNA)  then perhaos your body can learn to tolerate this " foreign body". If not, then perhaps that is the basis of the idea of the "evil twin" who may have become part of your personality.

I have heard of womb twin survivors who have two very different ways of being themselves, and they switch from one to the other almost unconsciously. In one case I regularly visited a woman suffering with autoimmune disease  (MS) over five years.  She was sometimes sweet and rather pathetic and sad but at others aggressive and raging. Two people in one. Was she a chimera I wonder?

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