Are the fingerprints of identical twins identical?

(istockphoto.com)

Identical twins are identical because they have the same DNA. They have that same DNA because they come from the same fertilized egg, which splits and grows into two embryos instead of just one. Such twins are also called monozygotic twins.

If DNA completely determined the structure of your body, then identical twins would be truly identical, and the answer to your question would be yes.

As it happens, there are inheritable factors other than DNA (we'd need a long detour to explain them, but suffice it to say that DNA isn't everything), and the development of your body depends in part on external factors.

Fingerprints form before birth, but are influenced by small differences in what the fetuses experience in utero. Fingerprints take on more or less definite shapes around 13 weeks, and while twins will often have some matching features to their fingerprints, they won't quite have identical ones.

Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.