We receive many questions about zygosity, that is, whether you and your twin are identical or fraternal. We hope you find the following pages helpful. If you still have questions that aren’t answered here, send an email to ws.twinregistry@wsu.edu.

• Identical vs Fraternal Twins
• How zygosity is determined
• Zygosity determination done by the Twin Registry
• The importance of zygosity testing
• Laboratories that perform zygosity tests
• Common misconceptions about zygosity