Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Identical-Fraternal Twinning
Name: Pat
Status: Student
Age: N/A
Location: N/A
Country: N/A
Date: December 2004

I read your explanation of conjoined/identical twins. When I had my 'identical' twins 20 years ago, there was speculation in the medical field that there is another state of twinsome they were referring to as "identical-fraternal", whereby one egg splits but is fertilized AFTER the split by two sperm. The result is 'identicals' that are not so identical. Can you elaborate on this?

In order for the egg to split it must first be fertilized by a sperm. Each resulting cell receives a complete set of chromosomes, half maternal and half paternal, so there won't be anymore sperm entering these cells even if they completely separate from each other to produce identical twins.

Ron Baker, Ph.D.

Click here to return to the Molecular Biology Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (, or at Argonne's Educational Programs

Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory