occur in about
but only about
once in every
occur in only
about one in
every 10 million
Did you know?
Medical advances have led to the discovery of another type of twin, known as tetragametic chimeras, where the embryos of two non-identical twins fuse together in the womb to make one person.
Just last month it was reported that a pair of unnamed American toddlers have become the first known semi-identical twins. They came to the attention of doctors because one is a hermaphrodite (with both male and female genitals); genetic tests revealed that both babies are "chimeras", having some cells that are female and some male. The most likely explanation is that the twins were conceived when two sperm from their father fertilized the same egg (a very rare situation). As a result in the embryonic stage they were briefly equipped with three sets of chromosomes. During gestation the embryos somehow shed one set of male chromosomes on a cell by cell basis, leaving them with identical genes from their mother but a mix of different genes from their father. Scientists knew that semi-identical twins were theoretically possible but, until now, had believed they could not survive until birth.
More Interesting Links
A 2005 New York Metro article about craniopagus-twins Carl and Clarence Aguirre, and follow-up photos of their progress since they were separated.
This article was originally published in May 2006, and has been updated for the
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