A Staff Report from the Straight Dope Science Advisory Board

Do twins have identical DNA?

August 2, 2001

Dear Straight Dope:

I just read Cecil's reply to Kraig Griebenow relating to fingerprints of twins being different. My wife and I were having a similar discussion, since I am a twin. She seems to think that the DNA is the same for twins. I have $20 riding on the opposite. Can you help?

Depends. How much of that $20 you figuring to send us? 

A key fact you fail to mention--always the way--is what kind of twin you are. As everyone knows, there are two kinds of twin, fraternal and identical. Fraternal twins develop from two independently fertilized eggs, so their DNA is different. Identical twins are a different story. If you and your sibling are identical twins, then you two were pretty close--so close that at one point you were a single cell together, a fertilized egg known as a "zygote." Usually a zygote, with its complete set of DNA, divides into two cells, then the two cells divide into four, the four into eight, and so on until a human being is formed (OK, that was simplified, but we'll be here for a semester otherwise). The DNA in each cell is identical. 

In the case of identical twins, the cell division goes a little too far--the dividing cells split into two separate clusters, each one becoming a separate human being. However, since there was only one complete set of DNA to start with, there still is only one complete set to end with, just split among two human beings. They are, in every sense of the word, natural clones. Since identical twins have the same DNA, they will also look the same, except for differences due to postnatal experiences, e.g., tattoos and scars, diet, etc.

A common but erroneous belief is that the two major twin types can be correctly identified at birth by looking at the placenta, the sac attached to the other end of each kid's umbilical cord.  It was long believed that fraternal twins always have separate placentas, while identical twins share one. While it's true that fraternal twins will have separate (although possibly adjoining) placentas, identical twins don't necessarily share. During cell division, if the cells split into separate clusters very early on, they may implant into the uterine wall separately, and thus develop two separate placentas. If the division occurs later (e.g., after the zygote implants into the uterine wall), then the two clusters will share a placenta. See Cecil's disquisition on the topic at www.straightdope.com/columns/000107.html (second question). 

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